Danger from the Deep is an OpenGL World War 2 German submarine 3D simulation featuring multilanguage support, multiplatform support, and advanced sound and graphics.
Danger from the Deep hasn't been developed in a while, sadly, so some modes aren't supported. You'll have to start by engaging in Convoy Battles.
Since the game is Open Source, we'd sure like to see someone run with it or join the team and bring this game to completion. While we see "activity" on the Sourceforge site, we haven't seen a new release since 2010. Regardless, this one is worth a look.