[seafarer]

Superstructure damage doesn't count for much. You need to put holes in the hull, at or below the waterline. Also, even if playing stock, you'll do more drastic damage if you make sure to spread your fish out - eg. try to put one under the stack, and one under a fore or aft hold (warships, try to put them under the turrets in hopes of touching off a magazine, but under the stacks is also good). Also, pay attention to torpedo depth setting. Getting hits nice and low on the hull is better then near the waterline.