Hi Robbie,
We booked all the trips with the rep when we arrived. (Dolphin discovery, Chichen Itza, Pirate cruise and Xcaret)
They were all freely available to book with a choice of days on which to go.
I am glad we did it this way. We had booked the Dolphin swim, and the day before we were due to go my sister had a really bad upset tummy

We just went and saw the rep and he rearranged it for a few days later. Not sure we could have done this had we booked over the net!
It really is your choice and I did think about booking before hand but am glad I didn't.
Have a lovely time.
Daisymayx