Water Pipes become available after Thermonuclear Cell had been researched, along with Water Pump. It must be said that without water pump, water pipe is entirely meaningless, so researching it before pump is... strange, to say the least.
Just like Heat Exchanger, Water Pipe will try to equalize the amount of water around and in itself. If the capacity of adjacent cells isn`t sufficient for that, they will be filled to the brim and the remaining water redistributed otherwise.
Water Pipe, however, can also pull the water out of Generators if there is any left after the tick expenditure. Usually, this is of dubious use, as the capacity of Generator is vastly inferior to that of a pipe.
NOTE: Water Pipe transfer rates decay when outputting the multiple elements, and with distance from Water Pumps. Thus, even with sufficient Pump throughput to feed water to Generators, they may not receive water quickly enough to stay full.