Leaving outside the fact that i'm not such a good pixel artist, I don't see too much of a "lack of skill" on that submarine. Looks pretty cool, something like a blueprint.
I wouldn't add almost nothing to it, knowing that it uses only 4 colors, but since is too tiny and simple, instead of simply make it "photorealistic" (in its own way...), it would be nice to give it more personality. I don't know the purpose of the drawing, but if you are planning to use it in something like a war game, maybe you want to make those (sorry the miss of thecnical terms) place were torpedos come out with more presence, as they almost disappear. Probably it would make it a llittle cartoony, but since it already haves outlines, there is not too much realism to lose.