Shorts him out. Electrocutes him, basically, to a degree proportionate to the amount of intake, but even enough to, say, make his legs go briefly numb would be a big problem if he's in the middle of the lake...

It's not just water, in fact, it's if matter and sprite energy try to mix in any way. Chewing or swallowing anything, liquids in the mouth mixing with his saliva... (There's an exemption for breathing, obviously, otherwise we'd never get anything done.) Goes the other way around, too, so don't try to steal his food.