An upcoming Type VII-C UBoat sim https://www.uboatsim.com/
Might be kinda cool. They seem to be going for realism and are building it all from the actual blueprints and have Friedrich Grade, lead engineer on U-96 and U-183 helping them with authenticity. They also claim it'll have celestial navigation. I don't think I have time for something like this in the near future.
What concerns me is it's listed as an MMO. And it'll have PVP, so Uboats shelling each other on the surface for some reason? I don't know what that's about.
It is not out yet, but it does have a demo.