A very interesting scenario. Lots of ways to look at it I think, all in your imagination of course. I can't think of individual examples in reality off-hand (other than U-505) but believe similar incidents did occur. I can see the crew being rescued somehow eventually, but by whom - friend or foe - might depend on the location and time during the war. The boat also creates an interesting dilemma: If it would be towed by whoever got to it first, or simply scuttled on site depending on the sea-worthiness. Then if the boat could be towed to a friendly port, would she be repaired and return to sea again? And with the same crew at that. You might check the hull integrity in your career/patrol logs to give you an idea about the boat's future prospects. I once scrapped a career when I returned to port with something like 9% hull integrity. Figured her patrolling days were over. Perhaps if you have enough renown you can get a new boat and still keep the same crew.
All interesting questions to consider but I'm afraid I'm not offering much of an answer. If it were me I'd probably work out some simple questions based on things like the above, then use a random number generator to essentially throw the dice to discover the fate. But that's just how I'd do it, ultimately whatever pleases you most. Maybe someone else also has a good solution to offer. Let us know what you decide!
