Shizuo had been slowly adjusting to his own merform, with his tail looking suspiciously like a great white shark's. Honestly he's not sure what to make of that, but at least it doesn't make him a real shark. Still, it was definitely different from swimming as a human, and when Kotori starts to flip over—

"Ah—!?"

He swims over to try and help, but because he was at a level below her he has to swim upwards. And due to his tail being of shark nature and his own strength, well... That is a lot of sudden propulsion and he's about to just collide into her instead.