I say where there's water (4)
I believe the answer is:
well
This clue is a double definition.
'i say' is the first definition.
(both are expressions of surprise)
'where there's water' is the second definition.
(a well is a water source)
(Other definitions for well that I've seen before include "Profoundly" , "and 24: Almost" , "four-letter word repeated in Shakespeare comedy" , "Healthy; extremely" , "Healthy - water supply" .)