Do you think Hydrogen is free or something? Worse, is that Hydrogen comes from water (at least that is the current cheapest way), meaning you get Oxygen and Hydrogen from it, but if you let the Hydrogen go, it will escape the atmosphere and be whisked away by solar wind (same with Helium). Then, you have just Oxygen, which is a green-house gas.

At least the ships release other chemicals in their emissions that decrease solar warming.

Ships are pretty good emissions wise because if the sheer amount of cargo they carry, even with burning the absolutely worst crap they can