Is this past tense? Could we also say 마시는 물? Whenever I review this I keep writing "drinking water" on my first guess
adding ~는 makes it present tense. ~(으)ㄴ is past and (으)ㄹ makes it future.
마시는 물 - drinking water 마신 물 - water that was drunk 마실 물 - water that will be drunk
Often in similar grammar patterns, 는 relates to present tense, ㄴ to past tense and 을 to future tense.
What you mean is drunk water, as opposed to drunken water which is water that has had too much alcohol...