Diluvion is out... already finished it!
http://store.steampowered.com/app/446790
Yes it's more of an action/adventure game than a sim but it has a fantastic story and graphics and some really clever ideas and it's very submariney! There are still a few rough edges so if you find you get frustrated easily it might not be your cup of tea but the devs have already released a patch that fixed the biggest issues with the camera and controls so if you like sub sims and Elite style exploration/trading games, this is kind of a steampunk underwater marriage of the two. Well worth checking out.