> Not to mention that a container ship’s deck is typically completely covered with, well, containers.

I guess in theory that could be solved either with huge removable panels around the containers (to be "put aside" somewhere during the loading operations), or placing containers with solar panels (and ways to deliver the energy to the ship) on the outer sides of the cargo.

Actually, maybe the batteries themselves could be loaded as containers on the sides of the cargo, with solar panels on them; that might increase the risk for the cargo if some catch fire, though

But, I guess it wouldn't be worthwhile.