Best is to keep low profile on each sonar ping, meaning you present your bow or stern to the sonar with +/- 30° at max. But if they get close they spot you anyway. At more than 170 feet you can go at half speed to get distance. If you don't hear pings anymore at slow speed it means they are more than 3000 yards away, you should be safe here, don't use your scope...
I find it easy to sink destroyers by letting them spot me, and let them come to 550 yards distance in which case I fire straight an aft torpedo. Success most of the time, after which I can escape on surface - or head to the convoy directly.
It's too bad that in CAOD, if you play without using the map, then you don't hear the ships passing over you when you are underwater, that's very annoying. I find no way to tweak the sounds to make all effects playing. I have eliminated the nasty 'Click' sound which will not cut the engine sound each time.
To exactly know about AI tactics on the hardest AOD level, I would need to mod the map to show everything while keeping the highest level which is not possible. But anyway: the AI is great and not repetitive, I have serious problems ending a patrol successfully when I get aggressive, the damages you take by getting shell hits are not a piece of cake
The flooding consequences are however showing-up too fast in order to have much choices to react: the sub will submerge automatically if too much water floods, that's ok, but after this you are doomed pretty fast, even if you go flank and blow your tanks, at some point you sink within seconds... Maybe the game is running too fast on Win7.