yeah - subs are hard to hunt and kill. . .
Instead of introducing complex or time consuming seperate find and attack rolls, we decided this was a better way to model it (since, almost opposite to surface engagements, destroyers are your best sub hunters and battleships are pretty much ineffective).
Things don't hunt and attack subs as effective as they defend convoy zones from subs (destroyers attack subs on a "1" but defend zones against sub attacks on a "2 or less") I think this is an important characteristic to model. Early in the war, when Churchill was Lord of the Admiralty, he advocated chasing subs all over the Atlantic like cavalry rather than escort duty - to disastrous results. destroyers were busy trying to find subs to kill while convoys were getting sunk at an alarming rate - it did not take long for the British to change tactics.
The game tries to reflect these strategies: when you don't have a lot of destroyers and planes - you are better off defending your convoy zones. Eventually, if you have lots of destroyers and planes, it might make sense to form hunter killer groups as well and chase the subs down.
anyway - my 2 cents - others have different strategies in dealing with subs. . .