Sharon
Assuming that the aquarium is open,(maybe someone can confirm that for you) I'd do it there.
Last year, my BIL and SIL did the dolphin swim at La Isla mall aquarium. (discount through Brant) It was fine. They got to swim with the dolphins, have their pictures taken with them and also the dolphins did the foot push with them.
The only drawback is that whoever else goes with the swimmer has to pay the entrance fee to the aquarium in order to watch.
I hope you and your family have a great time in Cancun. I look forward to hearing about your trip when you return.
Mike