The Mauna Kea used to have a place where they shone a huge light into the water. The light attracted plankton, and the plankton attracted the Manta Rays. We saw this many years ago. Since the Mauna Kea has been shut down due the earthquake,

I assume that they no longer do this.
Good luck, it is an impressive sight if you can see it.
Update, I just checked the website, and the Mauna Kea is re-opening on December 20, so they may start doing it again.
DD