So H2 fueled vehicles are just indirectly methane fueled, and not carbon-neutral.

Why not electrolyse water? It's not efficient, but if you use solar energy who cares? Set up a plant near the ocean and just let it run. All the inputs are essentially free.

Electrolysis uses a lot of electricity. Solar energy is not free, others pay money for the energy. What are you going to do with the hydrogen? It is better to use the electricity.

Green hydrogen only makes sense when everything is electrified and electricity is cheap. It might be needed for things like industrial heat or shipping where need fuel.