One of the best must-see experiences in Big Island. I saw other reviews stating the same but was skeptical. When the Manta Rays came so close to you in that night water - it is one of the unique experiences you will ever get. Having said that, please read ahead for best experience.
[Ignore the below if you are good swimmer and not worried about ocean swimming at night]
We were family of 4 with 2 good swimmers, 1 swimming pool swimmer (me) and 1 person who was worried about swimming (more so in ocean)
Since the experience explicitly said, swimming is a requirement, we got one life vest (Walmart has good one for decent price) ahead of time. I would highly recommend this for anyone who is not comfortable in Ocean.
Also, if you haven't done snorkeling before, get a rental or buy and learn to breathe through the mouth piece under water (Preferably in ocean). This will be very useful.
When you get into water, it will be choppy (most likely) and the board they offered was bobbing up and down. (it is same for all tour boats there) They keep the boat as close as possible (< 20 feet). And the staff watch you closely and ensure you are fine.
Once you hold the board, you are mostly fine. You can focus on using the snorkel gear and start looking down.
We saw at least 10 big manta rays coming very close to us. It is amazing experience and a big kudos to the staffs of this particular tour.