It's what's often referred to as "sim-cade". It's a mixture of simulation (like the Sonalyst games) and an action game, which, is very much like its spiritual successor, MicroProse's Red Storm Rising game "back in the days".
You have to base your decisions on pretty much the same things a real submarine captain have to work with. You don't need to do your own sonar, or TMA, or such, except classify the contacts, and determine when to attack, when to run, and so on.
DotMod added a lot of content, including separate towed arrays (and towed array damage), as well as playable surface vessels (really a hack of sorts) and is still actively supported.
Best online players? Wolfpack345, MagzTV, SorcererDave, and yours truly, online as youtube.com/@captchang
