Thanks for pointing me in the right direction! I think, having read all that stuff and more, that I'll leave the setting where it is. The fact that (in SH3) you can dive immediately has to be compensated for somehow

and those statistics (only 5% of all U-boat targets sunk were sunk using the deck gun!) suggest that vanilla SH3's deck gun was way overpowered (I was getting more than 50% of my kills from it in the early stages of the war).