Re: Why a submarine?
in theory a submarine on or near the surface would potentially be A NAVIGATION HAZARD. Subs have specia navigation lights for when they are on the surface (including an amber spinny one as I recall) for that very reason. You only see about 10% of the actual boat above the water line.
Subs are notoriously inefficient for cutting through surface water. Their hulls work well UNDER water but they are deliberately heavy with ballast. Attempting to use cargo in place of ballast is BEGGING for problems.
And do not forget a fuel per mile calculation, which is EXTREMELY likely to favor a standard displacement hull over a submarine one for fuel efficiency.
Subs are good for war and spying, because being under water you can HIDE. And unless you plan on carrying LOX with you (for a hydrogen-based propulsion), you would have to snorkel a lot. And snorkeling in places whee using subs WOULD make sense (like under the polar ice cap) may not even be possible (like under the ice cap).
I spent nearly 4 years on a U.S. Navy sub. And I wouldn't use a sub for cargo. Nope. For FUN, sure, to cruise underwater for touring reasons, looking at fish and corals and things, or for exploration (like finding ship wrecks or new forms of life) but that's the extent of it.
Robotic surface ships are a possibility though. Yet I think people would still want a captain on board to at least be available in case of emergency. And the total value of the cargo would justify it.
And contrary to what some may believe, stormy weather still affects a sub at periscope depth and down to about 150 feet or so. You need to get a lot deeper than that to be unaffected by heavy seas and stormy weather.
there are fuels for torpedo engines that can operate in a high speed engine without air, but the ones I have read about exhaust nasty substances (like cyanide gas) and therefore should not be used outside of military applications that make even BIGGER messes (like blowing up ships and other subs). And with 'green hydrogen' as the fuel, you'll need some kind of oxidizer (even for a fuel cell).
Additionally I have read that the USSR experimented with placing rockets on the forward part of a torpedo so that the rocket exhaust would shroud the torpedo in a gas cloud, giving it some thrust but greatly reducing the friction of the surrounding water. They were apparently VERY impractical to use, though.