Bottles are the best way to go, because then the water doesn't get icky. If you do use a bowl, make sure the water gets changed several times a day. (Rats will not just drink the water, they'll also play with it, use it to wash their faces, dunk their heads in, dip their tails in, etc, so it gets dirty quickly.) The ceramic bowls they sell for cats are good. I use those as food bowls, and the rats can't really move them around or tip them over.