Propane in the small tanks placed in vented spaces is dangerous enough. We all see often enough pictures of fires in marinas where propane was involved.
Imagine having hundreds/thousand of pounds of it as main fuel (which would be the logical use of a Bloom Box on a boat: main propulsion). Where do you put it? how do you vent it so there is no accumulation of leaked gas? Any fire on the boat would be a calamity.

Only hydrogen would be more dangerous.