You can steal a copy of "Silent Hunter: Commander's Edition" for $10 at CompUSA. That game is absolutely addicting. Real simple command interface - all you need is a mouse, really. Everything is represented by pictures, so you don't even need to speak English (although it helps to understand your crewmen when they call out destroyers). The premise is that you're an American submarine skipper in WW2 hunting the Japanese convoys in the south Pacific and around Japan. Lots of good action there. Good for 15 minutes, good for a couple of hours. You can even make your own quick missions. The graphics are older, but its amazing what they could do with games before 3D cards came around. Bonus: the video walk-through of a US submarine by a US Navy admiral and the short essay contained in the game guide.
The sequel - "Silent Hunter II" - is good too. The premise in that one is that you're a German U-boat skipper. I haven't gotten as far with that one as I have with the first one, but from what I've played so far, the sequel is much more challenging. They did a good job with it.
I've also tried "Jane's 688(i)" and "Sub Command". Of the two, "Sub Command" has better play. A lot of what was in "688-(i)" is also in "Sub Command", so there's some duplication there that can be skipped. Just get "Sub Command" and you won't miss anything.
Check out
http://www.subsim.com and you'll get a lot of good information from the guys over there. The guy that runs the site is also an active volunteer in the restoration efforts of the US WW2 vintage submarine in his home town.