The idea I proposed (based as close as possible to the actual events of the sub war), was that during a combat round in which any subs declared they are submerging, they could only be removed as a casualty from hits by Cruisers and Battleships if the hit roll was a “1” otherwise the hit would have go on a non-submerging sub or any other ship, or be lost if it could not be assigned. Enemy subs still hit normal during this round of combat.
Effectively, if you are attacking only subs in a seas zone, your capital ships could only hit them if they roll a one for the single round of combat they get to attack them. This assures a fairly high survivability for subs at sea when attacked.
I have tried this in some mock trials, and it does make USW viable, as it is much harder to kill subs. Also gives the Allies a reason to buy their own subs (escorting transports with friendly subs became a common tactic to stop USW).
It was a simple idea too.
Kim