Hmm, If you're at PD IRL they don't drop, they just Ram, or if you're too deep for that, they will drop a pattern, which will then explode underneath your boat, driving you to the surface, and then ram, if that doesn;t kill you they unload everything that can fire bullets at you, up to and including the captains sidearm.
This will then make your boat resemble a: a sieve (which sinks it) or b: swiss cheese, (may not sink but definitely won't dive safely)
at this point, for you, herr Kaleun, the war is over
In game they will drop DCs which will only be a problem for slower ships like Flower corvettes and armed trawlers. I have seen Flowers blow themselves up during GWX testing, I deliberately provoked them into it.
but not after that, because when I play the game for real I try not to get in such a situation.