A three-quarter-mile walk from the park entrance will leads to Waihi Falls, the pond where Kate and Sawyer came upon two submerged corpses still buckled into their airplane seats. Also, they discover a Halliburton case that the marshal escorting Kate back to the mainland had carried onto the plane. The case contained several pistols and also the small toy plane that has long been the object of Kate's obsession.
The underwater scenes however, were not filmed here. These were filmed at the dive pool at the Hawthorne City Pool in California. (source)