"Water that was drunk"
우유를 마신 would modify/describe something rather than being a stand alone noun phrase. E.g.
우유를 마신 아이 - the child that/who drank milk
Same with 마신. E.g.
마신 아이 - the child that/who drank
마신 can be used to describe what was drunk too though. E.g.
아이가 마신 우유 - the milk **that the child drank.
In the case of 마신 물, it's basically reads "the water that someone drank" which simplifies to "the water that was drunk.