I think its impossible.... The weight of the amount of water required to generate any amount of power (by means of potential energy) will be more than what the electricty u produce will be able to do work in moving the robot. That is, u need more water to produce more energy, but that makes it heavy so u need more energy to move the water.
Hydrogen fuel cell would work, Heaps of people have made homemade versions of these, but u would be producing hydrogen not electricity. U could use this hydrogen to drive a small generator or a tiny combustion engine like from an rc toy. I think that 's the only way u'll be able to use water alone to create such power. Check out these vids:
This will be pretty impressive too because its supposed to be impossible as pointed out earlier, however I think our concept of the conservation of energy has to be revised in this case.
We must consider the water as a fuel itself. I think thats where the extra energy comes from. They only use a 12v battery. These homemade cells really do work, I've done loads of research already. If ur interested check out a guy called Stanley Meyer (youtube his name) who made a car that runs on water. Many other people have done this, thousands, check out a a dude named Daniel Dingle from phillipines. Its just a suppressed technology.
Anyway good luk
mmm fuel cells.. Hydrogen fuel cells probably wont work due to the fact that the energy needed to extract hydrogen from water is greater than the energy which is produced from combustion of hydrogen.