One fish will not always do enough damage to show, also the hole may be below the waterline. When in exterior view go to the ship and dive below the water and look at the hull below the water, you might find the hole there.
If one doesn't give the desired effect setup for another shot and finish here off, not every ship in RL sank with one fish either.
Also if you light fire damage try this mod;
http://www.subsim.com/radioroom/showthread.php?t=180920