The biggest difference between the night dive and the night snorkel is your (vertical) location.
If you go on a snorkeling tour you will stay near the water surface, probably holding on to a flotation device that also has lights attached to it to attract plankton and thus the manta rays.
If you go on a diving tour you will be sitting on the ocean floor close and see the manta rays by looking up.
See this video to see the difference for yourself.