If you are going to go snorkeling through the shore excursions you can purchase on board the snorkel equipment will be provided in the cost in most cases. We brought our own along, about $30-50/person for snorkel and mask. I don't use fins but that would push you closer to the $50. Make sure you write you name on the equipment if you bring your own so it doesn't get mixed into the stuff provided on the excursion. Nail polish works well.