in my own opinion when DF users are submerged in water they do not actually lose their power. They just simply drown in the water that is why they cannot use their powers properly. Their powers are just weakened, not lost.
Im pretty sure thats correct. When a devil fruit user i submerged, I'm pretty sure the only thing they lose is the ability to manipulate their ability.
So yes. even if you slash buggy underwater, he will be chopped, and he wont have the ability to re attach but that alone wont kill him.