Note, I changed the title to be more descriptive of how this thread has evolved.
Fuel cells and submarines, just as fuel cells and space, are certainly viable alternatives for these unique applications which are relatively cost-unencumbered. On the other hand, fuel cells and aircraft (note the HY4 has virtually no cargo room) and, especially, fuel cells and cargo ships, are constrained by operating costs, chief among them being end-to-end energy costs (= 'efficiency' of energy conversion). I still haven't seen a good case for the latter.
Delighted to see the Nemo tourboat but can't help but wonder if a simple Lithium pack and electric propulsion would not be much less expensive to both install and operate? All they need to provide is a DCQC at some of the docks the tourboat pulls into to have it be a sustainable continuous operation. With their large length-to-beam ratios, tourboats don't require much energy in their everyday smooth-water usage.
EVs: 2 Wht/Blu SE Prem., '13 Tesla MS85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
EV Conv: 156v '86 Ram PU, 144v '65 Saab 96
Hybrids: 48v1kW bike
ICE: '88 Isuzu Trooper. Mothballed: '67 Saab (orig.owner), '76 MBZ L206D RHD RV