Apparently, any excursion, including rentals, are non-refundable once on board. DH and I had booked parasailing at CC before our trip. Then once there, DH decided to chicken out.

So we went to the Excursions Desk and were told it was non-refundable, but we could "exchange it". So, instead of paying the $150 for both of us to parasail seperately, we booked the Historic Tour of Nassau, and they only charged our account for the $46 total.
Anyways... all that being said, I seriously doubt you would even need to rent the stuff in advance. I am sure they have plenty, and on our cruise there were so few people using floats, tubes, or even the snorkel equipment that reserving them ahead of time is probabaly unneccessary.
Hope this helps!