How come we can't get a lot of energy out of a gl of water?

This question relates to Einstein's E=MC^2. M can be converted to energy and vice versa. Even a small amount of m would release enormous amounts of energy. I've heard that a gl of water contains enough energy to power NYC for a week or so. How come we can't draw that energy out? I know it has something to do with the energy that's put into getting this energy out. A chain reaction has to occur. What is it about plutonium or uranium that makes it easy to split an atom? Why not water or anything else?