There's no real reason why it didn't have ANY water, it's a question of how much it had.

With chemical/atmospheric/planetary/geological effects, you mix a couple of things together and produce something, mix them together in a different way with differing quantities/heat/pressure and you create something else.

Water's no different? Hydrogen on it's own burns? Oxygen aids combustion. Mix the two in the right proportions at the right temperature and pressure and what do you get?