Extremely fond memories of Aces of the Deep, due to it being one of the first "open world" games that I came in contact with. So, my experience is not from the scenarios, but from the career mode where you can start in 1939 and go all the way to 1945 ... if you survive the ever increasing technological race.
Though you can only play on various German submarines during WW2 in mostly the Atlantic, they are all detailed and have various "rooms" (like menu stations) for the player to do things in (like reloading a different kind of torpedo, or ordering to move a torpedo from the front to the back, or assigning repair work; also with time estimates by the crew).
If you play on a higher or custom difficulty, you will have to take into account a multitude of factors. Weather changes which increase or decrease the search area immediately around your sub, rolling seas may alter shots fired by your deck gun (which you can also manually fire: It's possible to destroy a target out to 8+ km even), fuel management (but you can also call for a meeting with a refueling sub), calculating firing solutions (and using a suitable torpedo), scanning capabilities of the enemy, air patrol areas with different kinds of planes, sea depth (to not get stranded on the ocean floor during a crash dive) ...
It's also filled with historical accuracies, like radio messages stating that certain events have happened (for example: a lot of last radio messages from other U-boats from around the time of their real last message), also certain battles of the ocean (like the hunt for Bismarck or the loss of HMS Ark Royal) are in the ocean area where they happened. You will see those ships on your and their way!
This game might not have the best look (you don't want to look at the "coastline" for too long), but there's a lot of immersion in this niche game,
And once you take out half a convoy by yourself while evading the enemy escorts, AND live to reach your home port, you'll want to do it again!