How much water would you bring along? Every liter is 1 kg you know, and our mechanical gladiators are limited in weight. Adding a pump and the rest of the setup, and it goes fast.
Most, if not all robots are rather water-resistent. It would be a very lucky hit to disable an opponent.
And of course, even if the rules would allow it- and those don't- the event organisers won't like it. Water on chipboard/mdf/multiplex floorboards tends to be damaging.