To expand on Penny's point, even if you book your airfare through RCI, they will not necessarily hold the ship if it's just a few passengers that are late. If it's a large amount of passengers, then they will. What that cutoff number is, I don't know.
I do know on my last cruise, I overheard someone on the walkie-talkie at Orlando Airport state that a United flight was delayed until 1:00 pm, and scheduled arrival was 10:00 am. This was again at Orlando Airport, with a 4:30 pm set sail time out of Port Canaveral, 45 minutes to 1 hour from Orlando Airport.
At our second port of call, which was Ocho Rios, Jamaica, I saw passengers with large suitcases boarding the ship, as we were leaving the ship to explore the port for the day. Whether or not those passengers were on that flight that was late, I don't know.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)