I
love my deck gun. Early in the war (and TBH I don't think I've ever played past early 1941) I prefer to use it if at all possible, either initially when first attacking if the situation permits, or as a followup to a first torpedo hit in order to finish off a damaged ship. I have way more gun ammo than eels, and there's nothing more fun than training a big ol' hunk of naval artillery on another ship and letting 'er rip.
Sometimes I order "feuer frei" and then go to external cam so I can really watch the boys at work.
The only real downside to this approach early in the war is if you're using the Q-ship mod and the poor lonely merchant you just opened fire on turns out to be a well armed decoy. Those guys? They shoot back!