What is the medical abbreviation for 'water' ?

aq

you can look this up online - by just typing in medical abbreviations to the search window

edit:

and I assume you meant medical, not scientific, medical for say writing on a medical chart or medical orders

And the medical community does not call water saline unless they are talking about saline for IV, they would never use the word water when they meant saline and vice versa.

ANSI designation is H2O

In the medical community they use the term "Saline" Which is water that is treated with electrolytes, hence NaCl2 or salt water.

Saline or H2O