Ooh! I've got a cruise coming up and was wondering the same about arriving at the airport early. I've done the Miami tour ex the ship to the airport which killed a few hours but did wonder about dropping the bags off in the morning and heading out.
I'm confused about when BA seem to have the 380 on the Miami route though, it seems to be changing back to a 747 sadly. Hope you enjoyed the cruise and didn't get bothered by Matthew!