Just my 2 cents… if your AP=0, depth charge will never give any flooding to the U Boat.
Furthermore, the splash damage max distance set to 6… means depth charge will not give any damage further than 6 meter radius of explosion…
To make it short, your depths charges will never damage any U Boat!
If you want realism, depths charges were very dangerous up to 50m radius, and they could tear U Boat hull & ballast up to 2,2cm steel
But that’s only my opinion…
