Water gets split into oxygen and hydrogen using energy. The hydrogen then gets burned to release usable energy, which creates water. At least as far as I remember from chemistry class ages ago.
There's some truth to what the gp said. Some hydrogen will escape, enter the upper atmosphere, and be blown away by the solar wind and thus be permanently lost.
I assume that this has been happening to all gases in the atmosphere for aeons, and thus, while technically correct, it is completely negligible for the relevant time scale.
vardump|8 days ago
dxdm|7 days ago