I spent a lot of time on Big Pine Key, but know enough about Marathon to give you a hearty thumbs-up on the place.
Marathon is one of the better places to live in the Keys, as its big enough to support a reasonable city infrastructure (restaurants, grocery stores, Home Depot, K-Mart, even a smallish hospital), and most tourists DRIVE RIGHT BY IT. That's right, the tourism "problem" is agreeably minimized in Marathon. Being (barely) within the Lower Keys, Marathon has fatter islands than the the ones up north; this equates to more living space, and escape from the Miami nuts that clog up Highway 1. There is a strong Coast Guard presence, and craploads of personal boats with potentially bad wiring (possible husband job caveat).
Keys living isn't cheap, but Marathon definitely offers more options than say, Key West or Key Largo. People who like amenities and "stuff" generally do not conform to Conch mentalities, which resembles the culture of a Caribbean nation more so than much of the United States. Just think Hemingway...
Marathon dive options are pretty good... run a search on this board and you'll find quite a bit (try keywords "marathon", "diving", "keys", "looe key", and "thunderball").
