They've got between four and seven hours to start riding down that river. Sooner might be better? It didn't sound like Estha had enough of the object-finding spell to use it continuously for like four hours of searching, but if they have time, they could stop at some of the more suspicious caves and look around visually or with the magic-detecting spell. 

Time to make a BOAT. 

Merrin has gotten really quite good at making random shit out of dried-and-resoaked strapwrack in a hurry. She can give Estha directions to help her, to speed it up even more, and then this is practically the optimal time of day for this, under the afternoon sun it's going to be bone-dry in like an hour max. At which point Merrin needs to at least haphazardly try to waterproof it a bit with sailweed leaves and duct tape, so it doesn't just turn into a mushy floppy boat as the strapwrack absorbs water again. 

...Ugh. It doesn't look like it's going to be the most maneuverable, though it's probably not worse than Estha bobbing around in his bubble-wrap flotation suit. (He should probably wear the suit, in case the boat has a problem, Merrin is really not as confident as she would like that the boat won't have a problem, it just seems probably still easier to concentrate while sitting it in.) 

On reflection she thinks she had better just swim it? She's a lot more maneuverable in the suit, and she'll have a chance of intercepting and nudging the boat if it's otherwise on track to hit any obstacles. 

She's not actually that tired - the three hours of mostly sitting on the ground and only working with her hands helped - but she's back to being spectacularly ravenously hungry. She can probably fit whatever's left of the food-production-spell output in one of her waterproof bags now that she's freed up all the space she was using for unassembled boat materials, so it wouldn't go to waste, if Estha were up for casting that now? 

Of course.  Create Food and Water.

(This 'dath ilan' concept, if it is Mariona's self-unadmitted fantasy, is saying disturbing things about Mariona's incredible depths of suppressed heresy... Esta will think about that later and find some reason to be less disturbed.)

It's probably kind of weird of her to find this as...romantic??...as she does? 

But Merrin is, more or less, having a picnic on the side of a gorgeous river after doing a scenic hike and then a Project with someone who, at the very least, LOOKS exactly like her boyfriend. It's a great date concept! 

It's pretty weird, but who exactly is going to tell her it's against the rules to have fun? No one! Certainly not Kalorm. 

...Yeah, okay. Having fun sometimes is good for her mental health, in the case where they don't find the Bag as soon as hoped and have to hunt for months. 

(Does that have...implications...for what the mysterious forces that planted both of them here want? Ugh. Probably. But chewing on that is not as fun as hanging out with Estha and eating bland but utterly delicious food on a riverbank. Merrin really will think about it, she's not going to try to just avoid it forever, Laeirthe does not need to nag, but - she's not going to let it ruin her picnic.) 

Merrin does not say anything about loud about the romantic associations of the situation but she does look cheerful as she eats and finishes packing up the leftovers in one of the waterproof bags, and helps Estha find a place to lower the boat into the river current and step down into it. 

Life demands many strange things of one.  This part would need to be a lot more unpleasant to count as a bad day.

(Esta is not aware that the two of them are on a date, on account of the Church of Asmodeus in Cheliax more or less lacking that gendertrope.)

He'll step down into the boat, wearing literally the most ridiculous outfit he's ever worn in his life.  Should the N̴̮̎̕ő̵̗̕ȯ̴͈͝d̴̡̺͒̑ḷ̶͆̎y̴̬̜͑̋ ̷̮̗͗Ö̵̧́n̷̝̬̑ě̵̫ have a sense of humor, this would be the moment the Church picks to scry him; but that is not actually a bad thing from his perspective, it is better to be scried than not.

It does not take long for them to reach a more noticeably intertidal zone with no shortage of caves. 

It's going to be moderately awkward to bring the boat to a stop without flipping Estha over into the water. Merrin swims alongside him in her power armor, and has no trouble keeping up, but she'll need, like, 200 meters of warning about any caves his geography-knowledge thinks might be worth stopping at. 

Merrin is going to leave spell-timing basically entirely up to Estha. He's an expert on - whatever their job even is, in his version of Reality - and maybe in his version of Reality so is she, but she certainly isn't now. 

He cast Locate Object from the moment they started moving downstream.  He has two regular Locate Objects, each 12 minutes, and one Extended Locate Object, lasting 24 minutes.  All have a range of [880 feet].  When one such spell runs out, there's not much point in starting it again until they're around [1500 feet] from where they were originally -- unless the whole trip is to last less than 48 minutes, in which case he should maintain solid Locating in order to not narrow the range he sweeps.

If they still have margin to delay the trip without running out of Location for sweeping the basic pathway, then it might make sense to try detouring into the largest caves.

Merrin is the one with mapping {software} and speed estimates; he's relying on her to say if they have the margin for searching caves at all, and how much margin, and then sure he can make the call on caves that seem particularly likely to have sucked in the Holding Bag and have more than 880 feet of radius to search.

They have about four hours budgeted - it’s conservative, but Merrin would like them to NOT be taking ANY risk of getting caught in the tidal bore even if there are problems. 

The river current speed is inconveniently wildly variable. If they ever do this again then Merrin will have great data! But right now she’s having to repeatedly recalculate and extrapolate how much ground they’ll be covering in an hour, knowing that they have to get almost 20 kilometers back.

The current is definitely not going to be obliging enough to carry them that far within 48 minutes. It starts out moving not much more than 5 kilometers per hour - a twelve-minute spell will cover a little over a kilometer, out of 18 - but Merrin does have an approximate suit-recorded map of "this bit looked like it was fast".

Prooobably Estha will get the most ground covered out of his limited-duration spells if they time it for fast bits, but the bag might be slightly more likely to have settled in a slow bit, so Merrin is pretty unsure on that? She will just do her best to keep updating him on her extrapolation of what distance he'll cover in 12 or 24 minutes, using the suit microphone-and-amplifier to somewhat awkwardly shout things to him while she swings ahead of the boat to scout out possible obstacles she might need to nudge it away from. 

They will definitely have time margin to look at caves, though it sounds like they can't be Locating Objects for anywhere close to the full duration of the trip, and it might make the most sense on this trip to just get recorded suit-camera footage and magical-geography-knowledge audio for Merrin to later turn into more detailed maps. 

Probably in this case they should use Locate Object along the fastest stretches, and maybe a final stretch closest to the beginning of where the bag was lost.  They could peek inside the surface of some caves with Esta running Detect Magic in case it happens to be right out in front, he supposes, if they have time to do that anyway.  But Detect Magic won't go through [1 foot] of stone, and probably also not some sufficient depth of water.  They'll need to recheck those caves anyway later.

It is somewhat weird to think that probably an [Outer God] has checked in prophecy whether any of this works, and made sure it won't work.  But Esta does not otherwise know what they are supposed to do, besides running through the naive procedure for bag-searching over however many days of work, until it eventually turns up the bag, sometime after the [Outer God] got whatever it was that It wanted.

(Merrin does not feel like her current strategy for reasoning about their bizarre situation is anywhere near optimal, but she's mostly falling back on poking at it, confusing herself immensely, and falling back on making plans on the basis of normal logic and logistical planning. It seems like trying to reason about where the Bag might be based on narrative tropes and/or the inclinations of an [Outer God] is either impossible because she has no model of how an [Outer God] things, or else counterproductive because if the [Outer God] can plan circles around them then it might just make the Bag even harder to find if they apply more thinkoomph to optimizing the search? ...Trying to figure out what the [Outer God] wants and let it have that sooner rather than later is maybe, in principle, ish, a way to get off this planet sooner? But it feels incredibly insanity-making and Merrin has no idea where to even start with that logic. She's not trained in that! She is trained in Exception Handling search-and-rescue, and 'person lost in a cave system and wearing a beacon detectable by her equipment only within a certain distance' is a training scenario she's literally done! Like, less dangerous caves, but still.

Going off normal physical-causality reasoning only: they do expect the Bag to plausibly end up in a somewhat different location every tide, but from that perspective it might actually be better if it's wound up in a cave and not being tossed kilometers up and down the river channel with every tidal bore. It seems more probable it would get wedged somewhere where it would be subject to only attenuated tidal forces, and it might - might - stay put long enough for them to grid-search the entire possible region. Of course, it might end up in some cave region accessible only through a hole too small for an entire person to fit through - if it's another six planetary months from now and they haven't found it, Merrin would be rather less surprised by that than she prefers - but they can incrementally increase the probability of finding it, surely, by being organized...) 

They can find places near intriguing might-go-quite-deep cave entrances where it's possible, with effort, for Merrin to guide the boat to the shore without flipping it over and drag it up to where it won't get swept off while they have a look. 

It's not easy. 15 km upstream of Merrin's camp spot, there's a lot less silt infill and a lot more rock, and the rock is apparently a more appealing surface for algae and bacterial biofilm, and this planet is so, so, so good at slime. The still-wet exposed intertidal rocks are literally steaming in the 50° C afternoon heat, and yet the thick mucilaginous slimecoat is still holding onto an impressive amount of water. Everything is quite remarkably slippery to try to walk on. 

...And these same biofilms can also survive being exposed for many consecutive hours at winter temperatures that must drop well below the -40° C that Merrin recorded on arrival.

There's honestly something really inspiring about the sheer biological ingenuity of all this life surviving in such an inhospitable place. A team of actual dath ilani scientists could learn SO MUCH given a year to study this place and bring home samples. There must be SO MANY cool biomolecules that have no reason to exist in any form of life native to dath ilan. 

Merrin kind of feels like Estha would not appreciate hearing about all of that right now. 

Caves? 

Some of the organisms Merrin is spotting are ones she's never seen before! What a cool and fascinating set of microbiomes separated by only a few kilometers!

...She does not take any samples because they have. Other priorities. 

(The cave interiors out of the direct sunlight are even more full of slime and difficult to move around in. Merrin, in her armor, is going to have a much easier time of it than Estha in his stupid flotation suit.)