The Science of a water-powered clock
Just like the Potato Clock, the water here helps to create a very simple battery.
Inside the four holes on the top of the clock are bits of zinc and copper which do not quite touch. Zinc is the anode for the battery - that means that it has some "spare" electrons that it can loose into the salty water. The copper acts as a cathode which means that it can receive the electrons.
Of course, to get from one to the other, the electrons have to move around the circuit and moving electrons is electricity!
The circuit here can produce about half a Volt, which is just about enough to power the small LCD clock here. This kind of battery is not enough for most things though (a typical digital camera needs about 6 Volts).