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.

I am also following this with interest. It looks to be quite ambitious, but also a very long way from early access, let alone release. Until recently, it was pretty much a one-man operation, but I think the developer has been able to line up a publisher, so hopefully he will have additional help.

The demo of the working engine room is quite impressive, with virtually every valve and control faithfully modeled. I think the MMO part is to incorporate eventual Wolfpack tactics, but I have no idea how the game hopes to utilize PvP, unless he is also building detailed ship models.

The problem is, realistic sub combat is quite tedious in actuality. In single player games, time acceleration is a big help, else you could be looking at hours to setup and execute a single attack.