[Gratitude doesn't even begin to explain it. Ryuji's entire kit was made up of offensive and support ability, so even if he had his persona with him, it wouldn't have been much help. If the situation were reversed and Dave had accidentally gone into the water... man, he wouldn't forgive himself for a second. He can only make the people around him stronger, not cure them. Doesn't have versatility like Ren does.

The reversal of his injuries happens all too familiarly, like it's been done so many times that he can barely count. He's been knocked out, had his life drained to the brink of death, been shot at more times than he could count. It's only because of his friends that he ever stood a chance of making it out alive, and as he feels the pain start to fizzle away like a can of soda going flat, he closes his eyes and smiles appreciatively.]

Thanks, man.

[What else does he have to say? That Ren's always bailing his ass out of trouble? That he's sorry for being reckless, again? It's underpinned beneath everything that he is, and he knows he doesn't have to say much more than that for Ren to get what he means.

Now the question of how they explain this crosses his mind as he looks at Dave wandering around the pool, sizing it up.]

Stupid... goddamn monkey has to be as much a stubborn ass as me. [That probably makes no sense.] Yo, Dave! What're you.

Ohhhh.

Shit, this is gonna be cool.