I think the easiest and best solution might be, once you pass the conning tower interior on your way down to the command room, the only thing rendered should be the sky and ofc weather (because even if ur in the command room or conning tower it's the only thing you can see) and not display ships water etc., this way FPS might get a huge boost ;)

Rendering of the whole inside would me major stupidity. I guess there are some mess with shadows/lighting and shaders and not the polycount. Or maybe all is tesselated and not poly reduced;>