u use a script, go into ur map go to edit, then scripts.... id have to look to be sure but its water level over time with damage is what ya use ill look if i have time
or you could do it if you have units in an area, firstly make an area.
***If <local player> has units in area ------------***
and true
***then water1 (just an example of what the water is called) decreases its level from 7.00 to -1.00.