I was lucky enough to get the chance to go to the Big Island of Hawaii recently, and snorkel at night with giant manta rays. We went with a company called Sunlight on Water. Off the coast on the Kona side of the island there are two locations where you can snorkel with mantas - one by the Four Seasons and one by the airport. Dive boats take scuba divers out to the sites while other boats take snorkelers. The divers get on the bottom of the ocean floor, about 30' down, and shine lights up towards the surface while snorkelers shine lights down and float on the surface holding onto a big floating ring. The goal is to make a column of light in the water, which attracts plankton. The manta rays come and eat the plankton, more or less ignoring the divers and the snorkelers alike.
The experience was really excellent. We saw seven manta rays. Everyone in our group, of varying ages and snorkeling experience, was able to participate and loved it. The manta rays themselves are mighty impressive, with a wingspan of over 10 feet by the larger females, with the males being smaller. We had an excellent orientation and educational presentation by a woman who then videoed the experience underwater, and got incredible shots.
The manta rays swim beneath you, and do rolls so you can see both the top and bottom of the rays. There are also silver fish circling around pursuing the plankton. When the bubbles from the divers rise up, it is like a gorgeous underwater chandelier with bubble crystals and silver fish intertwined. So, even when a manta is not in view, it is something to behold.
I've never snorkeled at night, and never with manta rays. It was the highlight of the trip. If you get to the big island, give it a try! It was so worth it. The video below from the company we went with gives you an idea, but it's better in person! (I got no photos since I do not have a water camera).