Funny Ai for me too. I was approaching a convoy and got detected by the DD's. They did a really good job of maintaning contact with me and keeping me down, even threading through the ships in the convoy. I tried coming up to PD a couple of times and they would start pinging me.
I finally got in the middle of the convoy and used a merchant to mask my ascent to PD. I then procceded to sink two medium frieghters with three fish from the bow, and three from the aft tubes.
DD's? Stopped and flashed lights at each other, and didn't do anything else.
I probably could have surfaced and shot up the remaining ships with my deck gun. Kind of a let down. I was really looking forward to having to save my skin, because these guys were doing such a great job earlier.