Hapuna Beach State Park, Kohala Coast

Hapuna Beach has stunning turquoise water. As you walk down the path to the beach, the landscape is dry and plain, but the water beckons you. You can sit in the shade by the rocks on the south end in the morning and then go jump in the water. The waves are a decent size so it's best if you're a strong swimmer. The water is gorgeous! 

There are some picnic tables in gazebos on hills above the beach, but really the place you want to be is in the water.
The restrooms are closed, and there are porta-potties, so that's not the greatest.
You can walk above the lovely lava rock coastline on the Ala Kahakai Trail from here for 1.4 miles to the northern end of Mauna Kea Beach Hotel, where there is a dedication to golfer Arnold Palmer. 
Another great beach for a swim is Mauna Kea Hotel Beach, but it's harder to get a parking space since the public spaces are limited to just a few cars each day.

Directions

Hapuna Beach is located at  Old Puako Rd, Waimea, HI 96743. There is plenty of free parking, though it's a steep, hot walk down to the beach.

Click on map for interactive view