Overall looks pretty good, but here's two things I think could improve:
1. The front being totally flat is quite goofy and just doesn't fit the otherwise semi-realistic design. Real submarines have a round front usually, or some kind of rounded pyramid or wedge shape.
2. There could be a different color than brown for the main surfaces. Although real submarines do look completely brown sometimes, it would be nice to have some variety, at the very least a gray metal texture for the floor to break up the monotony