We moved to Oahu a couple years ago. We took a trip over to Kona and did this night manta dive with Big Island Divers. I was a bit skeptical, I usually shy away from these "too touristy" things. I will tell you, I've been diving for quite a while - this was one of the best dives I've ever done. We had 19 mantas show up for our dive, you could have told me there were hundreds.... there is a lady who shows up every night with her video camera, will sell it to you of course, but she knows the mantas by their markings. We did the 2 tank dive, we dove right before sunset (twilight, I think they called it) and we saw 2 mantas just hanging around the reef. As it got darker we heard splashing around our boat and the mantas came right in, before any lights or anything were put out. This was a great experience and we plan on hopping over to Kona to do it again. I *highly* recommend this dive, and Big Island Divers were awesome. They have warm showers for after dive rinses, and they provided us with hot chocolate after the dive... I know, you think, geez, you're in Hawaii. Still, for the time of year it is now the water is cooler and the hot chocolate was a nice touch!
