The 60-year-old victim was part of a group on a dive 125 feet below the ocean surface about 14 miles southeast of Moriches Saturday morning.
A Coast Guard public affairs officer says the man surfaced about 40 minutes into the dive. He says emergency medical technicians who were aboard the dive boat administered CPR before being met by a Coast Guard boat while heading back to shore.
The victim, whose identity was not released, was later pronounced dead at Good Samaritan Hospital in West Islip.
The 48-foot Lockness Dive Boat is based in Freeport. Their website says the vessel was scheduled to visit the Oregon Shipwreck.