My understanding is that snorkeling is confined to Barefoot Beach because they want RCI instructors supervising snorkelers - a liability issue. Also, while you can use the area if you have your own mask and snorkel they also require all snorkelers to wear a vest. They'll rent a vest for $6. I am not sure if personal vests are okay, but I'd think this would not be a problem as long as it was in good condition.
The mats rent for $10 and they go quickly. I don't recall noticing people using their own but can't see why this wouldn't be allowed.