Luffy has fallen into water before and been saved, he's not going to die, but Kaido thinks he is, that's the point.
Big Mom washed up this very arc after falling in.
This is at least more believable than Kaido whining about not letting Luffy go alive, and then seeing him be rescued by a flying Sanji and going "oh well", or just as bad- assuming Luffy will fall in and doing no check to confirm.