Auto start and stop water AC motor using these two circuits
This should work for you:

The float switch is normally closed when the tank isn't topped off, and the timer can then energize the contactor, filling the tank.

Then when the tank gets full, the float switch will open, de-energizing the contactor and stopping the pump.

I'm pretty sure you won't be able to get the relay to work using the water and wires as a switch, plus there's the danger of evolving hydrogen and oxygen from the water via electrolysis, so I'd go for a nice float switch and not have to worry about any of that.  

There are some nice, cheap ones on Ebay India.