Don’t knock it. Its a swimmer deliver vehicle. SEAL’s use them, too, but ours are better. http://en.wikipedia.org/wiki/Advanced_SEAL_Delivery_System
Something - a tanker, a freighter, a sub, carries near a target and the swimmer deliver goes the rest of way. Its pretty big, so it may have a long range, or a big payload. It would be kinda ugly if a frogman delivered a big limpet mine, to, say, a LNG carrier. Oil terminals in the Gulf could also be good targets.
good reason to toss in the odd depth charge