Wed Nov 16....0800 to 1800
Taking Ruths comments into consideration you might want to make an inquiry of the cruise line about current tender loading policies that would preclude you from that first off last on group.
Personally, and I love AC don't get me wrong, I just dont know that it would be worth the time and money for what might amount to a few hours out of a 10 hour day with a lot of trucking around...that beach side glow might disappear pretty quick in more ways than one.
Get to shore by maybe what? in reality 9:30 or 10:00?, taxi to municipal airport, eh 20-25 minutes maybe more?, 60 minutes to get in past security etc and score and board a flight to SP (considerably less if youre timing is impecable), a 20 minute flight, deplane, walk down and cross the street to say Ramons for shear ease (not my day trip recommendation by any means) 15 - 20 minutes. That's on island at beach beer in hand by about what 12:30pm? maybe later?
Back calculate return with a margin of safety of 5:30 and youre leaving the island by 3:00 to make it back on the tender at 5:30 assuming there IS a tender at 5:30. I just dont know man, and my estimates may be (proabably are) off considerably.
Admire your tenacity though! "GOTTA GET ME SOME BEACH TIME NOW!!"
EDIT: but of course the cruise ship may offer a tour over to the Caye and then you just have to ditch your shipmates and make it back to the AC pickup point on time!