@PrincessShmoo is correct (as she always is)! No pre-reservations, you just stop by the water sports beach area on
Castaway Cay to reserve. We didn't do the aqua trikes - we rented kayaks instead - but they looked like fun! There were also stand-up paddle boards, paddle boats, and small sailboats for rent, too.
I took a picture of the price board:
It's a pretty large area you can paddle around in, and it's also very shallow so you can see lots of little shells and sand dollars on the bottom. DD5 enjoyed leaning over the kayak and trying to spot sea life and shells under us.