A classic in the genre. The missions may be canned (that is, have no random variation aside from units reacting to you), but the sim gameplay itself is good and the graphics/presentation is still great for a 1997 game.
Gameplay revolves around navigating your submarine - an improved Los-Angeles class you can choose from the sign-in screen - and achieving objectives such as sinking enemy ships and laying mines. You can also upgrade your sub and your crew between missions with earned points, and change your weapons loadout while at port.
You unfortunately cannot access any proper tutorials in-game. There are tutorial missions, short tutorial videos in the game directory and a comprehensive PDF manual included, but for a submarine simulator
it's a rather odd oversight. At least the manual is good and the game is great well once you start getting the hang of it. Having the assistants enabled helps as well.
That said, it is a 1997 game. It may not run that well on modern systems, even if it runs well on mine. The naval combat packs it comes in are rather pricey, too; you should wait for a sale.
FYI, the Steam version is the latest patched version, 1.08, initially a re-release of the game by Sonalaysts in 2006.
The music is of special mention. It's around 8 or so MIDI tracks, and it's the sort of great suspense stuff you might hear from a more subdued Tom Clancy film. You can download it here
and get some of the songs from the game remixes in MP3 format from here
(they're quite good).
You can also watch the special promo/EPK video here