The pilot stated that, about 30 minutes into the flight, he was over the water about 15 miles to the nearest land, when the engine experienced a total loss of power. The pilot made several attempts to restart the engine, each of which was unsuccessful; he subsequently initiated an emergency descent. The pilot circled a private vessel during the descent and ditched in the water nearby. He and his passenger exited the airplane before it sank and they were picked up by the vessel. The airplane was not recovered.