The shore dives at Puako just south of the Mauna Kea are very nice, the south end being my favorite. The entrances can be rough so watch the conditions and tides if you go over the shallow reef. The very end of the road, where you turn on to the dirt road, down by the small pebble cove, is the better dive and entrance. Morning is usually better. There's also a black sand beach on the south side of the Mauna Lani and you can dive around the point - the beach and cove area are mostly sand - but it's been murky the few times I've tried it.