I personally wouldn't book anything before noon and generally try for a flight around 1:00 p.m. or 2:00 p.m. Yes, you generally will be able to get off of the ship on time and efficiently, get through customs/immigration quickly, board your bus in short order and be at the airport well before noon, but there are so many possible delay points that I don't want to take the chance of missing my flight.
Plenty of people will post about how they always book an 11:30 a.m. flight (or whatever) and have never missed it--and I'm sure that's true--but it only takes one problem for that track record to come to a screeching halt. If you hate waiting at the airport and are someone who is willing to take risks and accept the consequences, that's great. If you aren't, then I wouldn't take the gamble.