Johnson, 36, was one of 14 adults aboard the vessel Rapture on an ocean excursion that left Honolulu on Saturday, said Punahou spokeswoman Laurel Bowers Husain. The boat is due back today.

An inflatable rigid-hull boat connected with the Rapture had taken the teacher to a spot about 120 yards offshore from Shark Fin Rock, said acting Sgt. Nelson Hamilton of the Maui Police Department. Johnson entered the water to swim at about 10:30 a.m. yesterday, and after 30 seconds when he didn't resurface, the crew looked down and saw him sinking to the bottom, Hamilton said.

The unconscious teacher was brought up to the boat and crew members performed cadio-pulmonary resuscitation. He then was taken to Kaumalapau Harbor on Lana'i, where fire and police personnel were waiting. Hamilton said.

Johnson could not be revived and was pronounced dead on Lana'i. The cause of death was not immediately apparent. Hamilton said an autopsy will be performed either today or tomorrow.

Johnson joined the Punahou staff in January 2003 and was a popular teacher who taught environmental studies and marine biology, Husain said. He is survived by his wife and a young daughter.