What did I do wrong in this twin gun boat engagement?

I wanted to ask the experts here. I am using GWX 2.0 and I was on my 2nd patrol in a VIIB East Northeast of Ireland. October 1939. It was dusk around 1650 and I was tailing a merchant and warship sound contact. They were moving away at 0 bearing and even flank speed submerged I was not catching them. I surfaced to try and catch up and never saw anything. Next thing I know 2 gun boats from Britian were headed at me guns blazing. I don't know what kind of boats they were but the were like little PT boats with machine guns fore and aft firing at me. I turn into them and we are going head to head. I pop off all 4 fore torpedos shallow in a salvo thinking I may get lucky or at least make them dance. I came close but I missed. So I have my deck gun pounding away and I crew the AA gun myself and we take out one PT boat fast. I shot the gunners personally as I was almost close enough to read name tags. Both boats are on fire and I have some moderate damage. No compartment more than yellow and my damage crew is on it. One boat down and the other one is a fireball about to be 100% dead. Then I hear "men down on the deck" and we all die??

I know I didn't handle this correctly. What should I have done? What killed me? I'm sure it wasn't a torpdeo as the boat has it's stern facing me at roughly a 270 bearing. I wanted to ask you experts for advice on how to handle this type of situation in the future.

Thanks in advance!