The issue is range, as you hint at with your last point. Because of how hydrodynamics works, big ships/subs are more efficient than small ones, and so it's rela

The issue is range, as you hint at with your last point. Because of how hydrodynamics works, big ships/subs are more efficient than small ones, and so it's relatively hard to make a small sub with enough range to be useful (or even enough thrust to reliably make progress in the open ocean).