The Ahalanui Beach Park (formerly Pu' ala' a) is a great place to spend some relaxing time at. Found just to the east of Isaac Hale State Park in Puna, you won't find many tourists here.
The natural pool (with some manmade improvements) is heated to a comfortable 90 degrees(give or take a few degrees) and is situated right on the coast. You can swim in total comfort while watching the waves hitting the breakwater. Because of the oceans proximity, the water is a mix of fresh and salt water.
The day we dropped by it was uncrowded with just a few couples and families present.
An added plus to the trip is the wonderful drive on the Pahoe-Pohoiki Rd. This is a narrow(!) two lane(?) road surrounded by rain forest on all 3 sides - Left, Right, and Above you!
Of course if you're staying in Kona or Kohala, this is about the furthest destination on the island for you - another reason why you gotta' have a car.