DCL partners with local vendors to offer many of those excursions. So it's likely that the vendor isn't offering it, or, the vendor hasn't set their schedule that far our. Keep checking back, as things sometimes open up closer to sailing.
Just FYI. I signed up for the Castaway Cay Jet Ski excursion several times over the years. Not once did we actually get to go out due to wind/weather. If I recall, the time periods were between late-February and May. It was disappointing each time. However, one year Royal Caribbean did send us out on a jet ski excursion during a very choppy, windy period, and it was miserable. In retrospect, I think Disney made the right call.
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