Ships sail at the cost of static resistance in water. Larger ships are more efficient, once buoyant. There is a New Scientist story about a guy on guard duty moving a moored navy boat by leaning on it, for a long period of time.

TL;DR marine is the one niche where "we had to make it a lot bigger to hold the batteries" isn't actually a big deal. If you do this the right way, you still have heaps of volume for cargo, and solar cells on the hold covers.