Regardless of what time the ship docks in Miami, and regardless of whether you can be the first to get off the ship, there is still the issue of immigration. They have to clear the entire ship before a single soul debarks. One problem and everyone will sit there waiting. It doesn't happen very often, but we were on a recent cruise when they kept announcing via loud speaker would so and so please report to the purser's desk. I believe they also have to clear the crew as well as passengers, since crew members often get off the ships after the pax, so that they can shop for essentials oR just get a change of scenery. But the bottom line is that you have NO control over what time you actually get off the ship.
Add that to all the other factors mentioned here and your flight may leave without you. No worth the risk, in my book.