I understand that it is because it lacks the calcium, yet as far as I can tell, all that is required in a fuel cell is hydrogen and oxygen.
Water in hydrogen fuel cells do not produce energy. Water is the waste product of the hydrogen fuel cell. It is hydrogen gas itself which stores the energy. If we take the pure water and apply just as much energy as we expect to get from the hydrogen with electrical input, we produce the hydrogen gas. Nothing in fuel cells is producing energy. Energy is always conserved and it must come from some source. Right now, the easiest source of hydrogen gas on this planet is petroleum, which isn't really much different energy wise than burning gasoline, and is in fact less efficient to extract hydrogen gas than it is to extract liquid octane.
