Hydrogen is already a proven technology for transport. I believe London Transport had some hydrogen powered buses a few years back.
Given that submarines already produce hydrogen ( as a by product when making O2 from water ) the methods of making hydrogen are also proven.
Power can come from solar, wind or waves, can it not, or so I've been told, LOL.
Water is not in short supply, as the oceans are full of it and it costs nothing, unlike oil.