Of particular interest - Seeing as we are talkling about this on a naval sim forum,

- is Blue Emu's highly informative thread
Are submarines worthless? which explains how to use U-boats in a way that works very effectively.
This was written for HOI2, mind you, so I don't know how much of it is relevant to the first game. If you are still playing the first one I would advise an upgrade to the sequel with it's 'Doomsday' expansion. it adds lots of little things to the game. It is one of my favourites.