Some recommended websites and forums:
SubSim.com [1] Sub and naval computer games
sci.military.naval [2]
RobotROV [3] ROV group on Yahoo
Sidescan Sonar Forum [4]
Note, the old sidescan forum [5] has been closed but can still be searched and read.
Vulcania Submarine [6] Homebuilt subs, 20,000 Leagues discussion. Yahoo group.
Links:
[1] http://www.subsim.com
[2] http://groups-beta.google.com/group/sci.military.naval?lnk=lr&hl=en
[3] http://groups.yahoo.com/group/robotrov/
[4] http://thunting.com/geotech/forums/forumdisplay.php4?f=10
[5] http://www.thunting.com/cgi-bin/geotech/forum/search.cgi?action=form&forum=sonar
[6] http://groups.yahoo.com/group/vulcania_submarine/
[7] http://groups.yahoo.com/group/vulcania_submarine/