I haven't done multiplayer
From what I've heard it's fun- it's (I believe) just players running
wolfpacks against the AI though, don't think you can hop in a frigate and hunt a Uboat down.
There are a few issues (imho) - nothing can shoot torpedoes but your sub (in SP)
but it's (imho) the best Sub Sim done yet. And it's (kinda) realistic
In 1939/40 you can prety much sail a boat around the British Isles.
By 1944 you're lucky if you can get out of port

(It's not actualy that bad- but I have had to crash dive several times when leaving the Uboat bases)