My wife bruises on her legs without even bumping or hitting herself on anything. What causes this? She also drinks very little water.

Needs evaluation. Bruising very easily is often benign, but in some cases can be a symptom of an underlying illness, including blood clotting disorders and some cancers. Very large bruises or easy bleeding, e.g. Frequent nosebleeds, would be of particular concern. A simple evaluation including history, exam, blood counts, and labs to check blood clotting would be advisable, especially if the problem is new.
Bruising. Does she take Aspirin or blood thinners? Some medications cause easier bruising. Could also be a problem with a clotting issue or anemia. I would address this with her primary care doctor who can run some blood tests to make sure everything is okay. If nothing else, it will provide reassurance.