The most common and most likely answer to this crossword clue is the 3 letter word MED
I've seen this crossword clue in The Times
M
E
D
Buddy explains!

The straight part
We know that "water" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case water = med.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MED mean?
MED noun- (chiefly in the plural) Medications, especially prescribed psychoactive medications.
- Of or pertaining to the practice of medicine.
- Intended to have a therapeutic effect; medicinal.
- Requiring medical treatment.
- Pertaining to the state of one's health.
- Pertaining to or requiring treatment by other than surgical means.
- May; might