There are 2 concerns that come to mind with hydrogen fuel. One being the cost of manufacturing of the hydrogen fuel cells but advancements in technology will take care of that in time. Concern #2 would be the safety measures behind putting them in vehicles, which has already been done. A hydroelectic damn or a nuclear power plant setup with a hydrogen fuel plant using electrolysis is easily attainable and would not be expensive in comparison to the infrastructure and logistics required for petroleum distillation or lithium mining for that matter. I haven't researched whether or not sea water can be used as the isotonic solution in the electrolysis process but im assuming you have a few mile markers to climb over before that is feasible, with that said, it would still be less costly to the environment and would produce far less CO2, CO or NO. In regards to renewable energy aside from it's few short comings hydrogen is the way, the truth and the light. Also, as a side note, you would likely be able to condense the exhaust from such a vehicle and recirculate the water and reuse it. The only thing standing in the way of hydrogen powered vehicles in my opinion are the dark pools of the petroleum industry.