He's glad to see this guy's doing okay. Honestly, Luca's beginning to wonder if his swimming partner needs to breathe air at all, and that's when he sees the fish swim, seeming to move at Peter's command.

"Woah..." he says, impressed when the fish swim around him. Sure, he's seen fish swimming around him all the time. There were the fish they raised on his family farm, after all, but those fish had to be herded with a staff. "That's amazing! So I guess you're fine down here for as long as you want to be, huh?"

At least, he's guessing since this guy seems to have aquatic powers himself.

"That's really neat! It'll be nice having another person to swim with that won't drown."

Sure, this isn't the only person he's met that can't drown, but it's a rare enough thing that it's still a surprising treat.