Until you've done it, you can't possibly understand the experience. The largest of these rays was about 10-12 feet across. They do underwater ballet and swim past you upside down within a foot of your face. They actually brushed my wife several times. Manta rays are absolutely harmless but their mouths are awesome. They could swallow a basketball whole. Which is amazing since they only eat microscopic plankton.
When you arrive at Sea Quest they fit you in a wetsuit, mask and snorkel. I recommend the wetsuit with legs as it is warmer than the jacket style (the water is cold at night). They will also provide you with prescription lenses in the mask if you need it. They rent underwater video cameras which you will wish you had if you don't. Bring a towel and something warm to dress in for after. But don't bring a lot of extra stuff (e.g. no fins). They will store your gear in a waterproof locker on the boat but there is limited space. You can also bring your own mask and snorkel if you have gear you are comfortable with.
This tour has the advantage of being located right next door to the hotel that has lights to attract the manta rays. It only takes 5 minutes or so to get to the viewing location. The skipper gives you a thorough orientation. Then when he says "The pool is open" you just slide off the pontoons of the boat and swim over to the raft with the powerful lights. There are two crew members in the water with about 16 customers. They put a "noodle" under your feet so your legs don't dangle down where the rays will be swimming.
In just a few minutes we started to see mantas swimming along the bottom about 10-15 feet below us. I already thought this was an amazing experience. Then one of them swam up toward us, did a loop the loop and swam away on his back. I was totally surprised and wondered if anyone had ever seen such a thing before. Next some of the mantas swam under us about 5-10 feet away. More amazement. But then they were ready to start the show. For the next 40 minutes, the mantas took turns swimming across from left to right then doing the loop and swimming back right to left upside down and no more than a foot below us. I have to believe the mantas were enjoying this as well. Eventually we had to go back to the boat and the waiting hot chocolate. Did I mention the water is cold at night?
One tip: I had my GoPro on the extension stick I use to get closer to the fish for normal snorkeling. But the Mantas came so close I had to keep pulling it out of the way so the camera wouldn't touch them. If you bring your own camera, hold it in your hand do not put it on an extension.