We have done several manta ray snorkels during our recent trips to Hawaii and IMHO the Sea Quest tours are the best.
The boat is big enough to provide a quick and comfortable 5 minute ride to the snorkeling site, but the group is small enough so that you get in and out of the water quickly.
The guides are good at making sure everybody is safe and enjoying the experience and that the mantas are safe. They are also very knowledgeable about the mantas.
The snorkeling itself is just about the greatest snorkeling you will ever do, hands down. We just about continuously saw the mantas swimming by, doing somersaults near us, coming at us with their mouths open.
At the end you get some hot chocolate and cookies to warm up.