It'll all change again when autonomous submarines get going properly. With no need to keep the jolly tars fed, watered and oxygenated you get loiter times in years, a flexibility of form factors and function that really is limited only by how smart your computers are, and low, low cost of entry.
(The only thing stopping this being as much of a funfest as UAVs are now is the fact that developed world navies think they're an employment bureau, a factor encouraged by governments. Won't cut much mustard when the balloon goes up - or, most probably, down)