Hello ddrgn, first of all thank you for your great job.
It is a very important mod for single player. However, some days ago I played a multiplayer mission with two friends of mine. One of them was detected after few minutes. Half of the destroyers haunted him throughout the game. Shortly after, my second friend was detected too and the other destroyers attacked him. Then I was able to attack some merchant ship easily, hehehe.
Soon, I was surprised because the merchant ships spotted me (opps, my periscope!) and I was shot by them but no destroyer was approaching me. Then, I started heading toward my friend and when the merchant ships were far and the destroyers very, very, near I emerged my u-boat. All the destroyers were attacking my friend... as if I were not there. I could even shoot a destroyer to help my friend with the deck-gun.
The most incredible moment was when my second friend was sunk and "his destroyers" attacked to my first friend... I was there... I was on the surface above him and the destroyers ignored me. The result was all the destroyers were attacking to my friend. Here you can find a screenshot:
Did you test the mod in multiplayer with several human players? It would be a pity that it is not possible to play this game in multiplayer because some u-boats can be invisible for the destroyers.
Thank you for your attention.