Water is actually a very poor conductor of electricity. It can only conduct electricity if there are impurities in the water. If im not mistaken, some of these impurities ionise in the water, and it is the movement of these charged solutes, and not the water, which causes electricity to be conducted.

Water also can only conduct electricity in the liquid state. its also fairly reactive. since its a liquid, its a pain in the ass to handle because it needs an airtight container, etc. no, theres no real reason to use water over copper except for the differenc ein abundance.