Sorry. Go back and study thermodynamics again. If you have the energy to generate hydrogen, you might as well use that energy to move the ship. You're always going to spend more energy generating the hydrogen than you will get back burning it or combining it in a fuel cell. The one exception is if you start from just the right feedstock to generate the hydrogen from, like some hydrocarbon. The leading candidate now is natural gas. However, in that case, the byproduct is CO2 -- so what's the point?

Or, maybe you were thinking of hydrogen as energy storage? Batteries are far better than hydrogen as far as that goes.

Storage for hydrogen can be really large, in the TWh size. Because they can use natural gas storage tanks. There's nothing that big for batteries. The big Tesla solar installation on that Samoa island was six megawatt hours, with 60 battery packs.

I'm going to have to disagree with you there. Electric cars are absolutely a thing; if you pay close attention you will spot a decent number of them in a day's worth of driving. Dedicated supercharging infrastructure is already fairly widespread, and basic wall electricity is already everywhere and is available as a second resort.

So sure, hydrogen cars themselves may exist, but the infrastructure does not, and I'm not aware of many people who reasonably expect that it ever will exist. Electric cars seem to have already won in the competition of "what renewable will replace gas".

There are quite a few hydrogen cars driving around in Japan, and infrastructure there is almost rolled out. For cars only Toyota is starting to roll them out in California. Some are starting to be sold around Europe as well. I think they've only sold around 700 in the USA.

Natural gas pipeline and storage infrastructure can be used. Because an order of magnitude more gas can be stored as hydrogen, than in lithium batteries I think they will be in the mix.

I definitely think it's a better idea for larger vehicles like trains on lines which aren't electrified.

Or do these new trains use some kind of (electric) hydrogen fuel cell, that doesn't work like traditional combustion?

It's much easier for trains, and larger vehicles to bring the infrastructure online. Existing natural gas pipeline and storage infrastructure can be used. Probably only a big company like Toyota could pull off getting the infrastructure in place for car sized vehicles. So far they have only done Japan, and parts of California.

I've seen TWh storage being quoted. Which is quite a lot higher than battery systems. Sure, if you can use the electricity directly, there is no need for storage. However, if you need to store a lot of energy, and you need the ability to send it around then gas isn't so bad.

Time will tell if it all works out I guess. But from the systems selling now, it seems like it's now viable.

RIP Stan Meyer.

the storage - you need very high pressure tanks to store them, and while this can be done, this is not cheap and easy

the inefficiency of creating and storing the hydrogen. When you compress a gas a lot of energy is lost by the heat produced by the compression.

So a hydrogen fuel cell vehicle will be 3x less efficient in electricity consumption than a battery electrical vehicle.

Additionally hydrogen can use the existing natural gas pipeline infrastructure.

In times of high wind, there can be enough energy to power all of Europe/Japan in a few days. New types of turbines need to be put into production that can actually use this however. Current ones are shut down so they are not damaged. So far I'm only aware of prototypes, but it's being worked on.