I have an Enfield No. 4 Mk 1. Wonderful, beautiful very accurate rifle and better balanced than my M1 Garand. But the only time I would shoot at a U-boat with it is if I was inside the boat.
But note that on the Mosquito FB.XVIII they equipped it with a 57MM Mollins 6 pounder AT gun for antishipping, but left the four .303 machine guns. I assume the .303 were for dealing with the gun crews on the boat's deck.