Ships at sea can distill seawater too so that's no big deal 7 if necessary they can grow greens in vertical gardens powered by electric light. Food, IMO is not the issue of staying submerged for long periods of time ; its the human costs of being social isolated fro long periods of time basicallyy on high levels of war alert. This is why subs have to port & refresh crew more than supplies, IMO, Also , IMO, this is why the US & I'M sure the Chinese are working mad on unmanned subs nuke powered that can stay submerged for a lifetime and whose systems can be activated in microseconds.
Perhaps this is why the Chinese have been mapping the ocean bed off the coast of WA and in particular its ocean trenches last year. It fine to have artificial islands instead of aircraft carriers which make sitting duck targets but it would be much better to have unmanned subs hidden at dept within cooey of the enemy in an ocean trench. If I were to be a Sub strategist, that's what I'd be planning. Such underwater automated/deep water subs would be immune to space spying too!
I guess Sub warfare will get serious when there are subs full of human breeding stock submerged for years to avoid human extinction on the surface!
