This could supply water — what do you think? (4)
I believe the answer is:
well
I believe this is a double definition.
'this could supply water' is the first definition.
(a well provides water)
'what do you think?' is the second definition.
(well, what do you think?)
(Other definitions for well that I've seen before include "Very much; healthy" , "Extremely; plentiful supply" , "Bore; extremely" , "Healthy; extremely" , "Flow up" .)