Good morning to you!
Two days ago, my dad and I booked a JetSki shore excursion from Carnival's website for Half Moon Cay. We don't book excursions very often, but whenever we consider them, we usually book online in case spots sell out quickly (which has happened - especially on Royal) before we go on-board. Luckily you can cancel for a full refund up to two days before your cruise departure in case you find out weather conditions may not be comfortable. We NEVER book activities with 3rd party.
I don't mean to go off-track on you, but do you plan on doing any specialty dining reservations too? I booked online too (in favor of my parents' anniversary) in case time slots sell out before getting on-board (I noticed some slots got closed out too).