how about this?
place a roundel under a sub on the board to signify its submerged. it can only move 1 space per turn submerged. you must use ASW search roll of 1-2 to detect sub from destroyer or cruiser. surfaced sub can move 2 spaces per turn. ASW roll is 1-4 when sub is surfaced. now, on second turn, if sub remains submerged put second roundel under sub. beggining of 3rd turn submerged sub must surface for 1 turn no matter where it is. even if it has to surface strait into hostile ships and be attacked.
not a bad idea. The one benefit to having them off the board is the surprise factor.
Great ideas though…