They don't say it explicitly, no, but if they're using a fuel cell on hydrogen and oxygen, it's got to put out water. That's how fuel cells work. (I admit that's another piece you have to already know, though - that or the fact that 2H₂0 contains less energy than 2H₂+O₂. I read through the thing carefully, though, and their descriptions implicitly rule out all the processes that could provide energy.)

Thanks for the info on metal hydrides, by the way - I didn't have any idea what they might be doing, but that's a likely option.