Hi,
I had implemented the same project last month and its working very fine..!!
I have used a fire sensor which will give high signal when it will detect a fire.
What I have done is, my robot is continuously rotating in search of fire and when it detects it, I have connected submersible pump operating at 12V DC (However consuming much current) through relay. On detecting fire, pump will be ON and it will throw water from that pump.
If you want, I can send video..!!