We had an amazing experience with Coral Reef Adventures and the nighttime Manta Ray snorkel. Huge shout-out to our awesome guides: Eric, Jana, and Shawn.
If you do the nighttime snorkel, you'll be hanging onto a board and not actually swimming anywhere yourself. With that in mind: 1) Don't bother bringing fins; and 2) Take Eric's advice and use a wet suit (they provide them, or you can bring your own). Even though the water is warm, you're pretty much just floating there the whole time, so it's easy to get cold. I was quite comfy in my full-length 3mm web suite.
We were lucky enough to see FOUR separate Manta Rays: 2 large ones, and 2 smaller ones (1 was a baby, according to Jana). They get up pretty close and personal! We had a wonderful time and would highly recommend Coral Reef Aventures and their Manta Ray snorkeling experiences! (We did the nighttime experience, but I'm sure the daytime ones are just as fun!)