I loved the vibe of Subwar 2050 as well! I should try to get my GOG version to run.
In my opinion the manual does the trick, it really works for world building. Microprose had a knack for this, one of my favourite Microprose games (A.T.A.C.) had an awesome manual, too.
The thermal layer mechanic is really great, as you said it’s like a flight sim turned into a stealth game. It totally fits the underwater setting.
