Yes the Med was difficult as hell in real life
...some reasons:
1) Merchant targets scarce and mainly the smallest crafts, not worth a torpedo and not even able to trigger the warhead detonator sometimes due to the shallow draft
2) Clearer water means easier spotting of submerged subs at shallow depth (Mainly from the air).
3) Air cover almost in any corner due to smaller distances
4) Main targets were fast and well escorted battleships and cruisers
I'm currently reading the recent "U-Boats in teh Mediterranean 1941-1944" by Lawrence Patterson and it really puts you in the mood
I heartly recommend it