We just got back from the Wonder and only rented fins for Castaway Cay. They were $9 for adults. We brought our own mask and snorkel.
If you have your own mask and snorkel, I would suggest bringing them. The gear there was fine - but you would be more comfortable with your own.
We did not bring our own fins - we thought they would take up too much room in the luggage.
Have fun!