Kaldur hasn't moved by the time Rain gets there. He's not immediately visible, having found a corner to hide behind, but his heartbeat is probably audible even through the water.

The problem will be reaching him, as he's picked one of the deepest points he could find without leaving the cavern. 

Mhm. Mhm. Yes, he sees.

If Kaldur isn't visible to Rain, then Rain isn't visible to Kaldur. So he can take a minute, remove his shoes, take off his shirt...

... and then, once all that's done, he looks at Batman. He smiles brightly.

"You have an underwater breathing thingy, right? Can I borrow it?"

"... I'll be expecting it back," says Batman, but he hands over the 'underwater breathing thingy.' "Undamaged."

"Yep!" agrees Rain, brightly.

He judges where precisely he needs to aim towards to land right on Kaldur. Yeah, he can make that jump. This decided, he takes a few deep breaths, expels all of the air from his lungs, puts in the underwater breathing thingy, and leaps. His aim is very good.

No oxygen in his lungs means no buoyancy, which means that even without an advantage in water, he is sinking like a stone. Right towards Kaldur.

Kaldur spent two years in the Shayeris military and has the reflexes to prove it.

His instinctive reaction to something dropping on him from above, even in a context where he isn't expecting an attack, is to hit it. Fortunately for Rain, he doesn't have his Water-Bearers to hand at the moment, so he's only punching with bare fists. Less fortunately, since Rain decided to dive feet-first, his blind swing connects with...a rather sensitive area. 

Ow, that's pain, that's sure some pain, ow ow ow ow ow—

—he has the presence of mind to lightly tap Kaldur before he shuts down entirely, and then ow he really doesn't have the presence of mind to make it back to the surface, ow ow ow ow ow.

He's sure glad he got that breathing thingy from Batman, because at least he is allowed to curl up and be pain itself for a while without having to worry about not being able to breathe underwater. Mhm. Yep. That was a good plan. Not that he can really tell right now. In case it wasn't clear: ow.

Kaldur recognises Rain after less than half a second and relaxes out of full alert.

It only takes him another half-second to remember that Rain can't breathe underwater on his own, which sends him right back into panic mode again until he notices the rebreather. Even with that, though, he should probably help his friend get to the surface. 

Rather than outright grabbing the person he just punched, he taps Rain lightly on the arm to get his attention. 

Rain's a little busy right now being agony itself, but - yeah, fine, hi Kaldur. He knows you're there. Good job. He makes a little thumbs up that could either mean 'I'm fine' or 'please get me to the surface.' It's not actually very clear.

(Elsewhere, M'kel finds Rain's commentary on the situation utterly hilarious.

But, ow, that was painful. Now she gets why Earth television always made such a big deal out of men getting hit there.) 

Kaldur decides to call that good enough; if he gets punched trying to help at this point, it'll only be fair.

He takes Rain's arm and starts towing him gently in the right direction. Not straight up—even with the water helping he doesn't want to try lifting a clone of Superman one-handed—but drifting upwards and in the direction of the ramp. 

Rain cooperates with this ferrying. Things slowly stop being entirely agony. By the time they reach the surface, he can think real actual person thoughts again.

He removes the rebreather, then coughs, "Good reflexes."

"My apologies," Kaldur says, holding out a hand to help him up. 

"I did not see you, and I simply reacted on instinct." 

"S'fine," he says, sincerely. He waves the rebreather at Kaldur in lieu of accepting the hand and getting up, though. "Return this to Batman for me please?"

Batman, for his part, is still updating the hangar's security systems. He looks to all the world like everything happening in this hangar is of utmost importance and is deathly serious about all of it.

Kaldur waits patiently for him to reach a stopping point, rather than interrupting. 

He doesn't know enough about technology to understand what Batman's doing, but it's interesting to watch all the same. Plus, this gives Rain a chance to get his breath back without making it obvious that Kaldur is waiting for him. 

"The grotto would have been better," says Batman, without looking up.

Kaldur blinks. He's not sure what he was expecting Batman to say, but it wasn't that. 

"...sir?"

Now Batman looks up. He looks at Kaldur, and enters Instructor Mode.

"Your opponent was trying to locate you through auditory perception, and unfortunately for you, water conducts sound. In a more open aquatic environment, like the ocean floor, this could have been used to your advantage to confuse where your location might be. But you didn't have one of those available. You had two major sources of water to hide in. This one is open, and as you noticed, consequentially open to ambush from above. The grotto is not. Furthermore, it's smaller, and has better acoustics."

Batman pauses, to see if Kaldur sees the implication. 'Better acoustics' seems like a terrible set of words when dealing with an opponent that tracks by auditory perception, but realistically, there isn't a way for Kaldur to have hidden from Rain's superhearing without better equipment and training. Instead, it would have been possible to go in the complete opposite direction, and create so much sound in an enclosed space that Rain couldn't detect where in the room Kaldur was. It probably wouldn't work either, Kaldur is sort of at a major disadvantage here, but it would have given him more room to work with.

"Thank you for the advice," Kaldur replies, looking thoughtful. Acoustics...Rain's enhanced hearing could leave him more vulnerable to deafness, or noisy distractions. This game is already proving its worth for their development as a team: it's useful to know a teammate's weaknesses so that one can compensate for them. 

He holds out the rebreather. "I believe this is yours?" 

"It's what I'm here for. Yes, it is, thank you." He then goes right back to what he was doing.

With utmost seriousness and gravitas, he adds: "Keep up the good work, both of you."

Rain smiles, then clears his throat and decides that it is time to just casually carry on as if he did not get punched somewhere sensitive.

"Can you use magic to find my sister?" he asks Kaldur, brightly.

"That...depends on how thoroughly she is hiding," Kaldur replies. "I am far from a master of sorcery."

This is probably not the best time to admit that he dropped out of his sorcery training to become Aqualad. 

"Better than I am! And she might not have thought to hide from you. After all, I'm the one seeking."

"True. Since you are not a sorcerer, she may not have thought to hide her magical presence."

Kaldur closes his eyes and focuses. The tattoos on his arms glow blue through the sleeves of his tracksuit as he reaches out with his sorcery, searching for Yvette. 

There are the subtle patterns of carefully woven magic over the whole base, distinguishable only because he's on the correct side of the magic hiding them from outside view...