Several that I know of, which I can name off the bat.
PT Boats: Knights of the Sea - I'm keen on this one. It looks good and has elements of detail I've only otherwise seen in the upcoming Silent Hunter 3, (mentioned above, also a game high on my list.)
http://www.3dgamers.com/games/ptboats/screenshots/
Dangerous Waters - modern sub/naval simulation by a company named Sonalysts, who made Silent Hunter 2.
http://www.scs-dangerouswaters.com/scrnshts.html
Midway - made by the developers of Nexus, a space sim game just released. It's a good ways off the horizon, but it should be out by their estimates at late next year.
http://www.3dgamers.com/games/midway/downloads/ (screenshots and movies).
Enigma: Rising Tide - It's meant to be a large-scale (massively, I can't say) multiplayer thing eventually, but it's out now. It is a good bit arcadish, but it looks alright.
http://www.3dgamers.com/games/enigma/screenshots/
Silent Hunter 3 is something I've been waiting for, a good while now. It shows a lot of promise, that's for sure.
Links will follow, as soon as I dig them up from the "intarweb".