Really Multiplayer
Make it a totally interactive game, if you can't find a full crew, you can use AI, but make the AI in such a way that humans wil always be better. So you advance till you are a cpatain of your own ship/sub, then you recruit people or they ask you if they can work for you and you run the whole thing. The way you asvance also helps you to know everything the ship/sub/whatever can do.
Something like that. Read what I mean btw, not what is standing here