Sounds like you have some great ideas for the game. Good luck! I'll definitely be following this project
There are a few free 3D submarine models linked in this thread:
http://www.subsim.com/radioroom/showthread.php?t=143576
It's an old thread, and there are a lot of broken links, but there seems to be some promising stuff there.
This weird site seems to have a lot of free 3D models too, in various formats:
http://artist-3d.com/free_3d_models/...hp&count=count