Gotham Harbor is gross. Everybody knows it. How could it not be, with so many villains using it as their de-facto dumping ground? Yucky water. If you can even call it water. Jason's surprised any fish still survive in all of that. But they do - don't ask how he knows that. Let's just say fish shouldn't smile, okay? It's creepy.
The point here is, you'd be crazy to take a swim in the Harbor unless you absolutely had to. Even then. So like, he's absolutely going to rag on you for this when it's all over, okay? You're getting teased about this for, like, a week, minimum.
And sure, you didn't jump in on purpose. Hardly. You got knocked in, mid-fight with these goons, and by no small blow either. So maybe he should cut you some slack, but that's just now how Jason operates.
Though between that hit, and what he's sure wasn't a soft fall into the water considering the height, Jason would've been a little concerned about you either way - enough to already be abandoning his own brawl to come try and back you up. But then you didn't pop back up out of the water. Not in five seconds, ten, thirty. And that's enough to make 'a little concerned' take a sharp dive into 'sorta panicked actually'.
So, in fairness, he's the one who jumped into the Harbor willingly. So maybe he's the idiot here.
He's still gonna tease you about it, though, damnit. That's what he does. So you'd damn well better be okay, so that he's got the chance to do that. He's gonna be real steamed at you otherwise.