This was an amazing and unique experience! I had read it recommended in every guide of the Big Island but was not sure due to the fear of being cold in the water at night. They provide you with wet suits that make a difference, and the reduced group is a big plus at the time of enjoying the manta rays.
As for the manta themselves... wow, just... wow. I kept saying "oh my god" all the time due to the amazement. We saw mantas almost all throughout the duration of the trip, that for me was perfect to enjoy it but not be a looong time in the water (it does get a little bit chilly after a while). At one point we saw about 5 at the same time. They are so beautiful and majestic. The guide that was with us kept telling us facts about them and I learned so much.
I definitely would repeat the experience if I come back.