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.