There's another option... You can rent fins for $9 a pair and bring your snorkel and mask. I don't think a snorke and mask takes up that much room but fins are HUGE. Personally given the choice I would rather only stick something in my mouth that has only been in my mouth (though I had no qualms about renting snorkeling equipment from disney, but now that we have our own mask and snorkel I intend to always bring it!)
Lisa