For now, yeah. Look at the bright side, in any convoy it's the absolute smallest target, you're doing your tonnage per fish ratio a favor

. Out of curiosity, were you shooting shallow, or under the keel?
I've fired a LOT of fish at them (default cargo) in the last could days and not one has CTDed on me (my test has been to fire 6-10 fish at them every test as fast as I can switch tubes).
Every one with ammo (on deck or below) has done it, though.
I experimented with magnetic shots, but they run under even at 1 foot and don't explode.