Of course, rats drink water because they are animals, and all animals require water in order to survive.
Rats will usually make their home adjacent to reliable water sources, such as next to a stream or river, or next to a human home where they can obtain water from the air conditioning condenser or spigots.
Rats can go a long time without drinking water, up to a week or more, if they are not very active, and still survive.
The chemical reactions that occur in the cells of the body require the presence of water, and that is why rats and all other animals need water in order to live.