Hydrogen is unlikely to see widespread marine use due to engineering costs and safety risks. It's more likely that merchant ships will run on ammonia, biodiesel, methanol, or LNG.
Long term I expect we'll primarily use nuclear and renewable power plants on shore to drive manufacturing of synthetic liquid hydrocarbon fuels for marine and aviation markets. That appears to be the safest option and won't require rebuilding entire major industries.
I think a more relevant issue is hydrogen embrittlement, it is definitely a problem for many alloys. I don’t think it’s a show stopper for fuel cells though, and it’s an important research topic that’s been worked on a long time