I thought it was a mini-sub
Might work for some spec ops stuff although I don't think it could of carried one...
BTW, the mainstay torpedoes for the USN during the war were:
Planes - Mk-13; 22.4" diameter, 13.5" long, weighed 2,200 lbs, 600 lbs warhead.
Subs - Mk-14; 21" diameter, 20.5' long, weighed 3,280 lbs, 643 lbs warhead.
Ships - Mk-15; 21" diameter, 24' long, weighed 2,840 lbs, 825 lbs warhead.
So as you can see they were all very similar in size and weight.