The pilot reported that about 40 minutes into the over water flight between islands, he requested a descent to 3,400 feet, which was approved by air traffic control. The pilot reduced the engine power to 2,000 rpm; however, the engine rpm dropped to 1,500. Actions by the pilot to restore engine power were unsuccessful. Following a successful water landing, the occupants exited the airplane and were rescued by the United States Coast Guard about 30 minutes later. The airplane subsequently sank and was not recovered.