Rats will spill or soil a bowl of water. An 8-ounce water bottle (the kind with a ball bearing stopper) can be bought at any pet store, large grocery store, or department store and is suitable for one or two rats (will hold enough water for a day or two). Buy a larger water bottle if you get more rats or need to leave them for a day or weekend. If using an aquarium for housing, make sure to buy a special holder or suction cups for attaching to the side.
If your water bottle drips, run hot water over the lid to help establish a seal with the ball bearing and the washer. Make sure to fill the bottle completely, because gravity-style water bottles work better that way.