I have done some research on this, and I doubt they will sell out - still I stringly suggest you get there as early as possible anyway because you want to get the most out of your time there.
I would be first off the ship and grab a taxi to the island. Have them drop you off at the entrance nearest the underground lagoon - that is the location of the ticket booth.
Please come back and tell us how it went.
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.