Hello,
indeed it already exists for a while, but it slowly becomes a serious sim. Just got a free software CD from a compuer magazine, with this game "Danger from the deep" on it. Deals with german subs of WW2, you can have a look at the game, and even download it for free. It runs on Windows and Linux, water graphics are great, ships not yet polished as in SH3, but ... promising.
Screenshot gallery is here:
http://dangerdeep.sourceforge.net/gallery.html
It is an open source project and everyone is invited to help and develop it.
Greetings,
Catfish