Panaluu black sand beach. It's pretty far south, but on the way to the Volcanoe National Park. Although we didn't snorkel there, I understand the snorkeling is very good there. Plus it's suppose to be a great place to find turtles because of the cool water seeping into the bay from underground. But we didn't have problems finding turtles, they were everywhere we snorkeled!
THe beach by the Mauna Kai resort. My favorite of the beaches in Kohala. I believe the beach is named Kauna 'Oha. You may have to wait to drive to the beach. The access is through the resort entrance and only about 75 parking spots are allowed for beach goers. We waited about 20 minutes for a parking permit (which is free), but it was well worth it. The beach and snorkeling is fabulous. Do be careful walking from the parking lot to the beach, though! I just about got beaned by a mis-directed (I hope) golf ball from the course adjacent to the beach. I understand for golfers this is one of the best courses to go to.
Hapuna Beach in the Kohala district. Voted the prettiest beach in Hawaii by Conte Nast. This particular day (on our trip around the island), there were high wind advisories. The only people in the water were body boarders. A few days later we went back and the water was swimable.
Punalu'u is a black sand beach just south of the village of Pahala on the southern end of the big island. Note that Pahala is not Pohala on the northern end of the island. The whole area down here was populated more heavily in ancient times since there were fishing villages on virtually every inlet and cove. Then the sugar industry boomed with emigrant labor and shipping ports resulting in even more population. Coastline destruction resultant from earthquakes and tsunamis, the demise of the sugar industry, and recently drought have all combined to turn virtually all of Ka'u (pronounced Ka oo) into the ghost district of the big island. It still gets traffic though, since people who rent cars in Kailua all come this way to see Volcanoes National Park. Some of them stop here. There is a rather nice resort called Sea Mountain (after the newly forming Hawaiian island still submerged) Golf Course that has lodging and golf at extremely reasonable prices for this type of resort in Hawaii.
'The black cinder sand beach at Punalu'u is situated at the head of a small bay formed by two rocky points. Atop the northeastern point, Kahiolo, are the ruins of a large heiau and a huge, flat-topped sacrificial stone. A small storm beach of black sand called Keone'ele'ele lies to the east of the heiau at the head of a rocky cove. The southernwestern point of the bay, Pu'umoa is the site of Punalu'u Beach Park (pictured), a county park complete with parking, showers, restrooms, picnic pavilions, drinking water, electricity, and camping sites.' ~ from Beaches of the Big Island by John R.K. Clark (the undisputed best source of beach info here)
Don't believe it if anyone tells you the Big Island lacks beaches. It actually has 80 or more, and new ones appear - and disappear - regularly.
In 1989 a new black-sand beach, Kamoamoa, formed when molten lava shattered as it hit cold ocean waters. Some beaches are just a little hard to get to - several are hidden behind elaborate hotels or down unmarked roads for which you'll want a four-wheel-drive vehicle or dauntless hiking spirit (or both) - and others have dangerous undertows and should be used for sunning and fishing rather than swimming.
In Kailua-Kona and even in Keauhou, it's true, there are no broad expanses of coral sand. The most beautiful, swimmable white-sand beaches stretch along the Kohala Coast. The surf tends to get rough in winter. To be safe, swim only when you see local people swimming in the area. Few public beaches have lifeguards or staffed beach centers.