It's been a while since I've tried a decent subsim, but after I've looked at the preview movie for Silent Hunter 4: Wolves of the Pacific I'm pretty sure that I'll give this one a try..
You can find a preview of the game here, from Gamespot.com, including some really nice screenshots and a couple of movies.
http://www.gamespot.com/pc/sim/silen...&mode=previews
Lots of interesting info on the official Silent Hunter 4 site here:
http://silenthunter4.uk.ubi.com/
Including a nice encyclopedia on the ships:
http://silenthunter4.uk.ubi.com/ships.php
And some impressive screenshots:
http://silenthunter4.uk.ubi.com/screenshots.php