If you are playing the American subs, I would definitely say to stay on surface regularly. If you don't have the Search Radar, on your turns go to decks awash and listen on your Hydrophones throughout your turn.
If you are playing the DLC and using a German sub, dive regularly to listen on the hydrophones, especially in bad weather. I can't express how many times I have almost run into escort destroyers and merchants in bad weather in the Typ. IX U-Boot purely from relying too much on my watch crew and not using hydrophones. That was a hard lesson learned, if you know what I mean!
