Consider a cargo airship operated remotely. The propulsion would be based on solar powered fans. Even if it took weeks to cross the ocean it would need essentially no cost to operate (or incredibly little) compared to operating a ship. If one blows up (hydrogen is volatile) you'd dump all the cargo on the ocean but you wouldn't be dumping tons of diesel fuel and the frame of the the ship is smaller than a ship. If you can do it without a crew you've essentially made shipping almost free except for loading and unloading at ports. AND you can put ports in areas that don't have access to inlets to the oceans or good shorelines. The most difficult part is the volatility of the lifting gas. You might have to load and unload cargo on platforms away from the coastline to prevent explosions. As for the hydrogen itself, the balloon can create it's own lifting gas by separating hydrogen from the atmosphere with an onboard chemical electric apparatus (presumably) if it ends up leaking hydrogen on its voyage. Lifting gas volatility has always been the biggest problem.