The Piper PA-28-180 experienced a power loss due to fuel exhaustion while flying over the Long Island Sound. The pilot was unable to reach shore prior to ditching. The ditching was successful and both occupants exited the aircraft without injuries. After being in the water for a while, they voluntarily separated and one was eventually picked up after being in the water for 3 hours. The other occupant drowned and was found several days later. The aircraft sank and was not recovered. The pilot failed to communicate his situation in a timely manner and even after telling ATC that he was low on fuel, accepted a descent which reduced his gliding distance. In addition, there was no flotation equipment on the aircraft. The pilot said he had planned the flight with a fuel burn of 8-9 GPH. The aircraft owner, who had purchased the aircraft 2 years prior, stated that he always flight-planned for 13 GPH when flying cross-country.