Well, to my surprise, so far the 'let's sail away and not tell anyone' theory seems to work.
Moving to the North side of the harbor where they have no submarine nets in place, I've snuck out and came clear of the harbor a couple of miles now, while the destroyer, which moved towards me a little bit further until it was kind of trapped in the corner at the end of the wall, is still stationary against the wall.
I really hope it does for a long long time. I did fire my last two torps at it but one was a dud and the other did put a hole in the hull but didn't sink her yet ... Seems I live to sail another day fellow Captains! Thanks for not giving up on me.