A wonderful Track killing simulator and probably the best avenue to start playing naval RTS before you move to games like Harpoon or Command: Naval/Air Operations. The addition of the Naval Warfare Project mod improves the game dramatically.
I wish it had faster time compression and a more accurate detection system. The ranges at which enemies are detected and indentified do not feel very believable, and the time compression feels too slow, especially if you're playing with no aircraft.
At the time of this review, the game is $5, and you get 2 other good naval sims along with it ('688(I) Hunter Killer' and 'Sub Command'). Or you can pay a bit more and get Dangerous Waters too!
I highly recommend Fleet Command, but note that you may have some issues with modern graphics hardware.