Like Mark said, the All Aboard time is NOT flexible! There are a couple of time zone changes on the 7 night cruises (2 time zones on the Eastern - if I remember correctly, and 1 on the Western). Make sure that you change your watches when you are supposed to and in the correct direction!!
The ship has left people behind before because they were not on board.
In Grand Cayman you want to be back to the dock before the scheduled LAST tender back to the ship.
Getting on and off of the ship is very quick and easy - especially when it's docked. When you have to be tendered ashore, it's still easy, just not as quick because you have to wait for the tender to make its round trip. (Always in Grand Cayman, very infrequently in St. Thomas)
Remember the Magic and Wonder are ships. They carry boats. The boats they carry are there just in case the ship fails.