Are ocean "supply convoys" going to be incorporated into AH along with the subs? Historically, WWII subs were used primarily against merchant shipping. Except for some surprise muggings, they were really no match for surface warships, especially in terms of speed (they were extremely slow when submerged - practically sessile).
As for surface attacks, I believe most U-boat attacks were carried out at night whilst surfaced.
For subs to operate efficiently, they would have to have in conjunction with airborne reconnaisance. Otherwise, you're just left puttering around a patrol zone, hoping for trade.