Hi,
It's my first post here, so be gentle
I must say that I'm enjoying the Battle of the Atlantic in CEAW a lot. I'm amazed how several simple rules can create such strategic dynamism which Hearts of Iron series failed to create for years with their industry, resource and stockpile systems. This doesn't mean that it cannot be better, though.
I'm opposed to static rules. There should be as much player involvement as possible. This is because the whole game is supposed to be centred around strategy and the strategical situation should be changing throughout the game. That's why I think that you should proceed with a combination of damage multipliers for unescorted convoys and invisible subs after attacks on unescorted convoys.
The damage multiplier thingy is good because it requires the participation of both sides. Currently even if the Allies ignore convoy escorts and are busy with e.g. intercepting Swedish iron ore, the Axis will not be able to do as much damage as one could think. The Allied income is so great and the convoy-sinking process takes so long that the Allies can be sure that some convoys will always get through even with ZERO escorts. This simple change would force them to care more about convoys IF the Axis is still a threat naval-wise.
The second rule (subs remaining invisible after attacking unescorted convoys) should be applied ONLY if the first one is applied, too. The reason for that is that bombers shouldn't be able to dominate the oceans on their own. Yes, they were a threat to the u-boats from 1942 on, but the Allies didn't suddenly stop escorting their convoys in e.g. 1943, when they gained the upper hand in the Battle of the Atlantic. Escorts were used for convoy protection throughout the war and it should be like that in-game, too. If the Allied player isn't doing enough to protect his convoys, then he should face the consequences.