Must visit Sting Ray City. Don't care if you think it is hokey of if you are concerned about getting "stung". Just go. It is so worth it.
I have a snorkel guide for my favorite areas. Google Testudo's Snorkeling Guides. http://snorkelblog.tumblr.com/
Cheaper eats: Sunshine Grill, Tim Buc Tuu, Cimboco, Dukes, Al Fresco, Kurt's Korner, roadside BBQ and Jerk/fish Fry's, Guy Harvey's dinner specials, Chicken Chicken!, EATS.