For marine applications, I think propane fuel cells will work better than batteries. Unlike batteries, energy density of liquified propane is good. Unlike hydrogen, logistics of propane is solved by now as it’s already widely used to fuel vehicles.

Currently people usually burn propane in combustion engines, instead of oxidizing in fuel cells. In theory, fuel cells are more efficient than combustion engines.

There’re commercially available fuel cells used for marine applications https://wattfuelcell.com/ however it seems low powered, not enough power for propulsion.

And there’re different ways being researched to synthesize propane: some biotech bacteria, traditional industrial chemistry with new catalyzers like trimolybdenum phosphide nanoparticles, etc.