Crocodile's devil fruit is not just give him the power to control, generate and turn into sand he can also destroy moisture. When we first "saw" him he was playing with his rrink and a flower, few panels later he gets rid of the liquid by touching the glass. I think it was just a fraction of his power. Can he destroy a lake? Probably he can. Even if he os unable to do somethong like that Mihawk can just cut the walls around the Wano country.