Hydrogen systems still being classified as prototypes after decades of research should tell you something. Nobody is making hydrogen boats because they’re simply inferior in just about every conceivable way. Cars got heavily subsidized, billions in R&D over decades, and are a failure. Hydrogen boats aren’t even vaguely worth trying to bring to market.
Economics uncovers bullshit, you can’t hand wave away the cost of infrastructure, someone needs to pay to create and maintain it. Solar can hit 2c/kWh for a few hours per day in a few areas of the globe, but now you’re both idling infrastructure and stuck with long distance transportation of Hydrogen and or electricity which has their own associated costs. Just do X sounds great, but doing X always has associated costs.
For comparison, including transportation and all other overhead bunker fuel gives you ~15c/kWh after you burn it, though this varies a lot. It’s horrifyingly polluting, but dirt cheap.
Scale alone isn’t going to fix green hydrogen it requires a fundamental breakthrough.