Cold Waters with the dotmod mod is fantastic Iron, so familiar to the Microprose Red Storm Rising back in the C64 days.
Silent Service on the C64 was great, and when SS2 came out on the Amiga, it made the ships have a solid model - first time I came up on a Japanese CV is something I still remember.
688i from Jane's was another great sub sim. Still has one of the best sonar models of any sub game IMO.
I went to buy Uboat based on your post Dolby, turns out I had bought it during one sale or another in the past, a pleasant surprise. Giving it a whirl on Sunday.