This was my husband's favorite excursion of the 7 we went on during our trip. The crew provides wetsuit jackets (very appreciated) for you to wear, and snorkel masks, though we brought our own. They cruised us out as the sun went down to a spot near Kona airport (which sounds awful but wasn't!) where the mantas gather at night.
The crew did a great job with a few things: they were careful to tell us that mantas are not dangerous and do not have stings like stingrays, or teeth like sharks. Basically, mantas are big underwater birds that are scared of us if we make sudden moves. Second, they made sure everyone was comfortable and had questions answered before we got in the water.
Getting into the ocean at night and hanging onto a board is a VERY strange experience. There are handrails, which were really appreciated, and we were all shoulder-to-shoulder looking down into blue-lit gloom. I admit, I was pretty nervous. Even having snorkeled before, the difficulty seeing the depth and the atypical posture of holding a handle and trying to float was unnerving.
All that disappeared when the mantas arrived. Watching them swoop up and do slow, graceful backflips inches from you as they scoop krill into their mouths is so beautiful. The first couple of times they came close it was a little scary, thinking they might bump into you or be startled, but these BIG creatures are so graceful and calming that I could have spent hours there.
The trip felt too short, but only because it was such a wonderful experience. I would recommend this to anyone who is interested in an unusual adventure, but I would NOT recommend this for small children. These are BIG animals, and for the safety of the mantas and everyone else in the group, having a small child panicking and potentially getting into trouble while trying to snorkel would ruin this experience for everyone. Let your children enjoy an evening in front of the TV and spend a beautiful evening with your significant other. The kids will enjoy this when they're teenagers, but probably not before.