you can book them at the 60 day mark and if there are openings you can book while on board. We usually wait for the castaway cay ones, float rentals, parasailing etc until we see the weather that morning. However we booked the dolphin Anguilla swim the first chance we could which was right at 8am on the 60 day mark. That one only has like 30 slots and will fill up fast I would imagine.
So my advice is that if there is an excursion you have your heart set on book it early you don't have to pay anything up front and can cancel up to two days before you sail.
It depends on the excursion you are booking. If you book in advance, you can't change onboard without a hassle. On the other hand, the popular (cheap) excursions book up quickly. So you have to be savvy to get it right!
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