The sheraton Mauna Kea still does this, we were there in april.

Granted the best thing to do is to get some binoculars since they are still far away - or take the boat trip, but i'm not sure how much you see.

They start shining the lights at 7 or 8 pm.

The boats then come and the divers and their lights attract the manta rays. After the boats have finished up the rays stick around. I'd go and see them at 8/8:30/9pm and there are no boats in the way. We went down there - the restaurant - the few nights we stayed and we saw two or three each night.