So far I've done 4 patrols, and I managed to sink a decent amount of ships, but only one with a torpedo. My 4th patrol was pretty tense, because I was constantly being hounded by destroyers. Everywhere I went I would run into one, with no merchant vessels in sight. I'd dive and move around silently to avoid, with depth charges going off all around me. Managed to make it to my patrol and finish the mission with very little damage though, but I only sank one ship.
One thing that annoys me though, is that you can only man the deck and flak guns if the weather is perfect. I'm the Captain, and if I need the deck gun operational when there are winds higher than 6 mph, then my crew better do it.
I also wish there was a way to make shift schedules or something, because having to manually switch crews around all the time is really fucking annoying.