ASW is not much of an issue in model warship combat. It is very difficult to build a submarine in 1/144 scale (the most common scale), and arming one is even more difficult. Making it reliable is even tougher, and less than half of subs that are sunk are due to enemy action. There are not very many at all, so most of the time you don't need to worry about them. Also, a submarine is not a particularly effective combatant because it is very slow. It needs either a disabled or very unlucky victim to score a hit. For simplicity, torpedos are fired by a contact button in the bow, though they would not be effective beyond about 6" anyway. Because the operator needs to be able to see his vessel, the advantage of stealth doesn't exist, especially because everyone is paying close attention to the uncommon combatant. Submarines also have very little reserve bouyancy, so if one springs a leak for any reason, even a leaky propellor shaft, it will sink rapidly. Also, my club (the WWCC) allows all ships to ram submarines.

Current ASW tactics in the WWCC mostly consist of ships trying to run over the sub to keep it under and unable to attack. It is also possible to sink a sub with gunfire, though the sub operator must have really screwed up to present a good target. The rules allow for a single cannon to be mounted through the hull to fire downward on subs, but amount of work for such a small threat means that nobody has done so. The rules also allow for a weighted line to be dropped on the submarine, making it unable to surface. Again, nobody has tried this yet. Most clubs have outlawed pyrotechnic devices of any kind due to the danger of mounting it on a very expensive, very flamable ship (and even worse: in the proximity of other very expensive, very flamable ships). I tend to doubt our insurance would cover us (nor could the club survive) if there were to be an incident.