The Kona coast of Hawaii is the only place I know of where night snorkeling/scuba diving with manta rays is a common thing. The mantas here are reef mantas (around 12' across). Several boat tours leave every evening to a couple of destinations where the mantas hang out (Keauhou Bay/Outrigger Resort and off the Kona Airport). I assume the mantas are attracted by the plankton that is attracted by the bright lights at those locations. We usually aren't organized tour people, but we went on one of the manta night snorkel tours out of Honokohau Harbour. We went North to the mantas off the airport. When we showed up, there was one other boat there. From the excited hooting from the people in the water, we could tell the manta rays were there. The way it works is you get in the water with snorkeling gear (but no fins, since accidentally kicking the mantas with fins could injure them). You then all hold onto rope loops attached to each side of a surfboard which is towed around by one of the staff (who does wear fins). The board has bright lights attached to the bottom which shine down and attract plankton, which attracts the mantas. You basically float limply on the surface and watch as the mantas do their looping rolls scooping up the plankton sometimes inches below you. There were about 5 mantas below our surfboard during the 45 minute tour. Sometimes they were so close, they would brush against the people floating on the surface. I assumed using a blinding strobe for my photos would not be appreciated by the snorkelers around me so I tried to just use the lights from the surfboard for illumination. The problem was, our surfboard lights were bright blue, which looked kind of unnatural. The neighboring boat's surfboard used white lights so some of the photos of the mantas from a distance were illuminated by those lights and looked more natural. I was also using a slow shutter speed to let in more light, so the movement of the mantas up close made most of the photos kind of blurry.
