If you loved the movie Das Boat, you will love this game. It's a submarine game simulation, it has some of the best graphics for naval games that I have ever seen.
I played my first submarine game called Gato on a pc junior back in 1985 or so. In Silent Hunter III, you play the captain in the German submarine fleet. There are several modes in this game, a tutorial which covers the submarine basics, Campaign mode, multi-player mode, and a musuem so you can see all the vessels in the game.
If you play the campaign mode, you choose a year and a starting harbor, and than you can set out on high adventure, sinking merchant ships of course, hopefully lightly defended.
You earn 'Reknown" in the game, the more ships you sink, and missions you do you earn reknown, and you can you this to buy submarine upgrades, train you're crew, get more experienced crew and so on.
There is some crew management, crews that are inexperienced suffer from fatigue on missions. Later in the war you get attacked from aircraft a ton,
Sinking ships is a bit difficult, having some basic trigonometry helps. I can't understate how nice the graphics are, the storms, you will suffer from sea-sickness if you watch the waves too long. Patches from Ubi-soft have made the game stable, I still have had a few crashes.
If you haven't already downloaded it, there is a nice mod called the grey wolves, it enhances the game a ton, its about a gig download, but worth it.