Actually, difficulty varied wildly depending on the ship class you were sent against. Battleships were easy, as long as you didn't surface. Patrol boats were on the other end of the spectrum: too small, too fast, and too manuverable to use torpedos against effectively, but they also had enough guns to make a gunfire action against them dicey.
My favorite attak sub simulator was Red Storm Rising, with scaleable difficulty and different periods of the Cold War available to play in. You could only play as the US, but you had a choice of submarines from the Permit on up to the Seawolf, and the number of opposing classes was vast. You could do some crazy unrealistic things in that game, like have 16 or more Tomahawk anti-ship missiles flying around from your VLS-equipped Los Angeles, but it was really quite fun to play, and I got very good at dodging torpedos. The only times I would take hits were when I got bracketed by at least three torpedos, I got too close to a Russian surface ship and he nailed me with RBUs, or when my towed array was busted and one of those pesky Kilo-class diesels snuck up behind me and fired from less then four thousand yards.
