Located within the 120-acre Wainapanapa State Park, Waianapanapa Black Sand Beach is not a good beach for swimming. The waves can be high and the shorebreak and rip currents strong.
Wainapanapa State Park is, however, a great area to explore. If you've never seen a black sand beach, this may be your best best on Maui, since it's right along the Road to Hana - about four miles before the town.
Be sure to take time to walk along the ocean bluff above the beach and cliffside right near the parking area. The views back towards the beach from the top of the cliff are impressive with interesting rock formations and sea caves.
Because of the strength of the waves, the area seldom looks the same the next time you visit.