Rats should have access to fresh water at all times, supplied in a water bottle. A bowl can be tipped over and soiled by urine, feces and bedding, so a hanging, on-demand water bottle works best. The water should be refreshed every few days, and the bottle should be cleaned well each time the cage is cleaned. Cleaning water bottles in a dishwasher cycle works well as the very hot water helps to kill bacteria. Never try to clean a water bottle by putting soap into the bottle because it is difficult to rinse the bottle thoroughly enough to remove soap residue.
In the summer, two or more bottles should be maintained, if possible. Store unused, full bottles in the refrigerator, and switch them throughout the day. Rats can only release excess body heat through their tails, so they can become overheated fairly quickly. If a rat does happen to go into heatstroke, he can be given an electrolyte drink like Pedialyte to help recover. You can also immerse him in lukewarm water and promptly dry him with a towel.
Rio Grande River and Sandia Mountains, Albuquerque