On reading about water batteries like the Japanese's Nopopo, I stumbled upon one of my favorite MIT physics professor, Walter Lewin's demonstration on the Kelvin Water Dropper.
Kelvin's contraption is intriguing indeed. Using gravity on a simple and smart circuitry to turn miniscule charge differences between two separate conductors into a kind of battery.
As for the Nopopo 'toy', it works on a different principle, which is similar to normal AA batteries. It can use liquid as simple as water instead of some other chemical as electrolyte and can be filled by user with water at anytime. Designed to be environment friendly, but many consider its low power a con.