My father manned a 4" deck gun from the beginning of the conflict......It was an ancient relic from WWI and training consisted of a dozen or so practice shots.
He informs me that from mid 40 onwards, the British merchant marine were often helped by the addition of Royal Marine gunnery crews.
Be advised, U-boats seldom fought it out on the surface with armed merchants because of the potential of taking damage.
A heavy calibre machine gun round was more than capable of penetrating a U-boats pressure hull, nevermind a 3 or 4" shell.
Once penetrated, it ceased to be a 'submersible'
