The United States Coast Guard is searching for a man and an aluminium skiff that went missing near Betio in Kiribati two days ago.
The Pacific News Centre reports that the 24-year-old remained aboard the two-metre boat while two companions went diving about 180 metres from land.
When they surfaced the boat and its passenger were gone, but the divers were able to swim to shore.
A Hercules aircrew stationed in American Samoa has covered close to 5,000 square kilometres in a four-hour search.
The Coast Guard is also using two Self-Locating Datum Marker buoys, which refine the search area by tracking current and drift patterns.