The waterfall at the Waimea Valley Audubon Center.
A three-quarter-mile walk from the park entrance leads to Waihi Falls, the pond where Kate and Sawyer came upon two submerged corpses and the Halliburton case ("Whatever the Case May Be"). Later Paulo found Nikki's bag with the diamonds and his nicotine gum in the same pool. ("Exposé") In season 5, this is the pond in which Hurley and Kate awaken after the crash of Ajira Flight 316. ("316")
The underwater scenes however, were not filmed here. These were filmed at the dive pool at the Hawthorne City Pool in California. (source)