Creation thinks of fall damage as something that grows linearly with distance.  This is in fact completely sensible, given the way that potential energy and kinetic energy work; though it's anyone's guess about whether that's why Creation works that way.  The primordial inevitables who set the rules sometimes got up to some pretty strange stuff.

So how far would you need to fall, to impact into water with a relative speed of 19 meters per second?

You will have been falling downward for a bare bit less than 2 seconds, for a start; after accelerating downward at 9.8 meters per second per second, as is one standard Golarion gravity.

Integral calculus too simple to bother with will tell you that if your velocity is increasing linearly with time, then your average velocity over that time is half its final value; extended over a bit less than 2 seconds, you'll have fallen a bit less than 19 meters.  The resulting calculation, if you do it exactly, works out to baaarely over 60 feet; which is unfortunate, because Esta will take an additional 1d6 damage that he wouldn't have taken if it had been 59.9 feet instead.  If that strikes you as unreasonable, go talk to the primordial inevitables about it.

The first 20 feet of falling into water do no damage.

The next 20 feet of falling into water do 1d3 of nonlethal damage per 10 feet.  Esta takes 4 nonlethal damage.

And then everything else does 1d6 of damage per 10 feet.  Esta takes 7 damage.

Esta has 87 hitpoints.

And a Heal banked.  That is part of the Worldwound loadout for 6th circles.  Heal handles a very wide variety of possible problems, and has no material component to make it any more expensive than the cost of the spell itself; meaning that Heal is hugely more cost-effective to do via 'actual 6th circle cleric onsite' rather than by scroll or item.  Scrolls are for things you need only sometimes.  If it's going to predictably be useful almost every time, it goes in the standard spell loadout.

Esta would've cast Heal on himself to get rid of possible other status conditions, if he'd made until next dawn and spell-prayers, but he did not quite deem it Wise to use it up on himself right away.

Uh-huh.

Anyway, the actual problem is going to be the rapid waters that repeatedly smash Esta into nearby rocks, thereby breaking his arms and legs and mildly fracturing his skull.

Which still would only be a very brief and temporary problem for Esta and his Cure loadout.

Except for the part where it's hard to use spells with somatic components when you have broken arms.

So!  Has he been knocked unconscious yet?

Do not be insulting.  This barely even qualifies as pain.

He's not swimming very hard, though.

Merrin saw enough of the first rock-smash for her rapid internal calculus to be concluding that this is probably...not...very survivable.

And she doesn't have cryo facilities on this STUPID PLANET because GUESS WHAT MERRIN DID NOT PRIORITIZE EVEN SLIGHTLY in the last six months.

(She can do emergency preparation for cryo with stuff she has, because that would very plausibly have come up in the training sims that all of those lovely crates of gear were SUPPOSED to be equipping her for. Or, rather, she could have on arrival. Now she's dismantled a bunch of her specific-use medical equipment, with specific uses that make no sense to ever need to use on herself, for their motors and wires and reprogrammable electronics that can form part of her automated cave-safety-and-climate control or be replacement components for power armor repair. There’s not actually any point of doing initial preparation that relies on "and then a helicopter shows up within less than six hours to take her patient to a specialized facility”.) 

That thought goes through her mind in under a second and is set aside because it's not actually relevant to what she does in the next sixty seconds. 

She flings herself off the bank with as much force as she can and...in fact falls around 19 meters into water. 

OW

(But Merrin was expecting this, and is landing feet first - a headfirst dive is just an incredibly stupid idea from that height even in armor - with her legs clamped together and arms crossed and chin down, and she has a sealed air supply, and the highly-engineered composite material and lining of her power armor do some impact-redirection. It's 'ow' but she's not injured.) 

Armor reconfigured to aquatic mode (it's mostly the retractable flippers, which you don't want to be extended when you hit the water feet-first with great force). Merrin gets herself angled so she's streamlined. She dodges a rock - she has a lot more maneuverability than someone in unpowered armor that is not designed for swimming in Extra Challenging Conditions, and also she's really familiar with this river, even if she normally rides the less intense post-tidal-bore current back inland. 

Can she see anything? 

Lol 

She's on the totally wrong side of the channel, and at least fifty meters behind where she last saw Estha - assuming he's moving at roughly the same forward speed as her, they are both in short order going to be, like, kilometers upstream of her camp - and her visibility is maybe five meters. 

Merrin power-swims diagonally across the river channel, not fighting the current exactly - its main direction is forward, she wants to go forward, she and the water are in agreement on that - but combatting the turbulence that wants to toss her around in random non-forward directions. 

And she switches on her very powerful Headlamp: Underwater Search Mode. 

(It's not as powerful as a searchlight on a helicopter, because even the cleverest engineering cannot really make that possible as one component of many in a suit not much heavier than Merrin herself. It's still drawing enough wattage to substantially reduce her active battery life if she just leaves it on for no reason. But she's going to be using it for, like, ten minutes max. If she hasn't found Estha in an entire ten minutes then she's looking for a dead body, not someone she can still save.)  

Even a Very Powerful Searchlight cannot just solve the fact that the water is pretty murky with algae and sediment and all the bubbles. But she can see a lot further than five meters, and it penetrates a lot more meters of water than the light of a Continual Flame skullcap, which will be a help to Merrin only once she gets substantially closer than this. 

(It's not part of Merrin's model that the light-up hat the man she currently believes is probably her dath ilani boyfriend was wearing is going to still be working. She's assuming it was a normal electronic-battery-powered costume accessory and is not going to appreciate all the forces or all the wetness involved.) 

She's now closer to the center of the river channel, which had narrowed to barely fifty meters at one point and is now actually wider and shallower again, in a section of riverbed with somewhat fewer giant boulders wedged deep in the sediment. Convenient for her; the water that's made it through that chokepoint is now spreading out and slowing down a bit and there are fewer obstacles to waste time and energy dodging.

(Or from Esta's perspective, to crash into.) 

It feels like it's been about five minutes but, in fact, it's been 42 seconds. 

With all the tossed-up silt the water is carrying, Merrin's headlamp can penetrate thirty meters of water, which is not quite enough to see either bank from where she is, but is enough to see clear to the bottom of the channel; the portion behind the wavefront is sloshing wildly but is on average only about five meters deep at this point. 

CAN SHE SEE HIM???? 

No, but he will see a light distantly piercing through the water, on one occasion when he forces his eyes open; and he will... sorta maybe slightly try to move in that direction by flailing his upper arms and upper thighs and ignoring the pain that comes with that?

Honestly, Esta has taken a lot of nonlethal damage at this point, and as much lethal damage again as that, and a few more knocks on the head.  Even he is starting to feel like it might be a good time for a nap.

- what was that? 

Merrin is actually pretty unsure if she actually saw anything, looking for "movement" in the water is kind of difficult when the water, itself, is still moving very fast even past the river-channel chokepoint. It might just have been a dislodged strapwrack holdfast getting knocked around. 

But it's a direction to try swimming in. Merrin will swim that way, angling her headlamp back and forth a bit, scanning the water as hard as she can for solid shapes larger than a strapwrack holdfast that might even be human-shaped. 

(The one-minute mark since Vicar Esta cast Air Bubble from his scroll passes, not that Merrin has any way of knowing this would be relevant.) 

It would've been nice to have the leisure to look at his pocket-watch and precisely time that, so he could inhale deeply just as the spell ended.

He will hold what breath he happens to have, though.  It's not a bad reason to stay awake a little longer.

Merrin is COMING she's ALMOST THERE - 

- nevermind she's briefly redirecting nearly all of her attention to not getting beaned by a collision with a boulder in her path, she was distracted and did not see it more than five seconds in advance - 

- she dodges, successfully, but it involves some very frantic underwater-gymnastics and now she is tumbling through the water, trying to reorient her sense of direction - 

The river is finally cooperating with her, here. Relatively speaking. The water speed is down to only 20 kph in this section of riverbed, and the bottom here is more gravel than silt, with less sediment to disturb and make the water murkier, and they're both now well behind the actual tidal bore wavefront - her maximum headland visibility is more like 40 meters - 

It's still going to take her another thirty seconds to be sure she has her eyes on him again, and then at least thirty seconds after that to actually catch up to his position. 

(She's counting seconds passing. It's going to be coming up to two minutes. Two minutes is not necessarily an anoxic-brain-injury length of time by itself - assuming that somehow he hasn't just smashed his skull open on the rocks by now and made that the least of the brain-damage-causing problems - but she cannot start CPR in the MIDDLE OF A RIVER and she's not even sure how long it might take her to reach the bank carrying someone -) 

He's mostly focusing on holding his breath, at this point, instead of performing the Strenuous Activity of trying to flail his broken limbs.

RAW says:  2 x CON = 26 rounds, two and a half minutes of breath-holding.

After that, Constitution checks starting from DC 10 and increasing in DC by 1 each round, to keep holding your breath, which is a couple of rounds if Esta is lucky.

He might not, like, totally get all of that, under these conditions.

RAW says you suffocate to death 3 rounds after you start breathing water.

Merrin will, perhaps, also not be able to actually reach him as efficiently as she would in less adverse water conditions. 

(Merrin is not aware of the existence of Endure Elements and is expecting him to additionally be incapacitated by the icy water, which is warmer than it was in late winter but "warmer" is still, like, maybe 6° C. At least her suit cooling problem is very thoroughly solved). 

STUPID RIVER!!!! STUPID OCEAN!!!! STUPID PLANET!!!!!! MERRIN HATES YOU SO MUCH RIGHT NOW!!!!!

She does, actually, reach him around 15 seconds after the point at which he failed the Constitution check to keep holding his breath any longer, but she's focused on grabbing him securely in a way that won't make the severe blunt-trauma internal injuries she assumes he definitely has immediately worse, and not on getting both of them to the surface within the next three seconds exactly, because from Merrin's perspective it has already been more than three minutes and a ten-second difference in when she can start CPR is going to make less of a difference to his survival odds than, like, if he has a spinal fracture but hasn't quite severed his spinal cord yet and she fucks that up. 

That worked??? She leapt directly into a 30-meter tidal bore moving at 70 kilometers per hour after a drowning person, and she did NOT get herself killed and is not even really injured (she's going to have some bruises for sure but nothing is broken) and she CAUGHT UP and GOT HIM and eeeee!

Merrin will allow herself one (1) second of internal !!!!!!!! and ✨ ✨ ✨ and a feeling that might be described as "wow she did not know she was actually that cool!" 

(I seriously don't know what's wrong with you and your self-image, imaginary Kalorm grumbles. You're like the only cool person in dath ilan who isn't related to me.)

Merrin is ignoring that, because she's still in the MIDDLE OF A RIVER and every second is taking her five meters further away from camp and it's now time to book it to shore as fast as she can.

(She can check for a pulse while swimming and carrying him, though)

Is it... not the case that any humanoid who can't breathe water, regardless of how long they were or weren't holding their breath before then, would die (presumably with associated cardiac arrest) exactly 18 seconds after they started inhaling water?

Because if not, Creation is confused!  Creation is pretty sure that's how primordial inevitables imagined biology ought to work, albeit possibly without ever having actually seen a humanoid.  If Merrin's sheer presence and force of belief is mandating Reality to have more complex rules than that, in this faraway place where the primordial inevitables may well be paying less close attention, she needs to go explain on her own how she thinks biology ought to work.

Well! In this case Merrin would be pretty unsurprised at the absence of a pulse, because she thinks he's been unconscious and submerged for three minutes!

But, in fact, her detailed and confident model of human physiology predicts that someone who has just lost consciousness and aspirated a bunch of water - and is already in respiratory arrest and will not spontaneously start breathing on their own if brought to the surface, which is an entirely plausible outcome within eighteen seconds - would still, from that point, take somewhere between one and three minutes to progress to full cardiac arrest, simply because they're starting out with some oxygen still in their blood and that doesn't immediately drop all the way to zero that's not how things work, and there's a gap between the degree of hypoxia that causes unconsciousness - the brain specifically is an energy-hungry organ - and the level at which the heart stops working.

It might go faster on a planet with lower atmosphere oxygen levels, for someone unacclimatized to it. It might take longer in cold water, because hypothermia slows metabolism and thus the consumption of the remaining oxygen in their blood, but of course that part of her model is completely failing to account for the existence of Endure Elements. 

(That being said, these are super adverse conditions, and whether or not a weak pulse is still present at this point, finding it when they're both being jostled around in the water is a pretty high Concentration check even for Merrin, and she's not going to try for much longer than ten seconds because her top priority is reaching the shore.) 

(More than half of Merrin's attention right now is going toward trying to spot a candidate location to get out of the river. This would not at all be a problem if she were on her own! The banks are easily climbable in her power armor! But they're steep, and currently really slippery from the spray that the tidal bore sent up when it passed through this area several minutes ago, and there's no way she's getting herself up to the plateau in less than ten minutes while carrying someone. That's way too long!! She needs an even-vaguely-flattish spot near the current water level that she can drag her unconscious boyfriend onto in, like, at most another thirty seconds. She can worry about everything else - climbing up the bank and getting out of the path of the still-risng tide, and then the fact that she's at least 1500 meters upstream of her campsite and the walk would be entirely over horrible rocky karst terrain - after she gets him breathing on his own.) 

(If that happens. Merrin is tracking that it might just be too late. He looks...remarkably uninjured, actually...he's obviously pretty bruised and bleeding from multiple abrasions after the collisions with rocks, but his skull isn't visibly deformed or crushed. She would be feeling cautiously optimistic if she had medical facilities. In fact, the situation is worse than that, because she packed for upriver exploration and cave-searching. She brought a bunch of first aid supplies that would make sense for her to use on herself if she were to be seriously injured - but still conscious and able to treat herself, there was not actually any point in planning much for situations where she ended up unconscious or completely incapacitated - and she did NOT pack for needing to resuscitate a patient in hypoxic cardiac arrest while completely on her own.) 

This horrifying metaphysics in which people are not either Alive or Dead but instead have a variety of internal properties and conditions corresponding to particular forms of gloop inside their body, such that they could be a little dead or mostly dead without being all dead -- well, let's be frank here, this is necromancer pornography.  Nobody like this Merrin person is going to be clericed by Pharasma anytime soon!

But if this is the dominant view on how reality operates, Vicar Esta is going to be only very slightly dead at most upon emerging from the water.  Like, the sort of dead where he's stopped breathing, not the sort where his heart has stopped, taking this entire set of horrifying metaphysical premises at face value.

Be it very clear, a standard Breath of Life spell will do nothing here.  It has been longer than one round!  Now if this 'Merrin' entity has some sort of Breath of Life-like supernatural ability -- for reviving entities who are, on her own alien view of things, particular kinds of partially dead -- that is between her, and wherever her god is now.