Are you talking about CSA? We booked the catamaran cruise at the front desk, shortly after we arrived. And if you enjoy that beautiful cruise it as much as we did, you are welcome to book it again if you have time.
As others have said you can't book it before you arrive, be sure to book it shortly after you arrive to be sure there is room in case the resort is full. Some friends of ours went one night, liked it so much they decided spontaneously to go again and were able to get on with just a couple hours notice, just depends on how full the resort is. If you like it and want to go again it never hurts to try.