Here are some good Submarine Simulation game sites

http://www.subsim.com/index.php Check this out, its an excellent site for addons, and other scenarios for your SH III game. GWX 3.0 Gold is out now.
This site also offers Naval news and Intel articles.

Pick up a copy of Dangerous Waters here and at other sites. I just didn't want to name them all.
http://www.strategyfirst.com/en/games/DangerousWaters/
This game will be part of my inventory pretty soon. It looks like a ton of fun.