At about 1323 eastern time, a Mooney M20J crashed into the Atlantic Ocean off the coast of Florida after the pilot reported medical difficulties in-flight. The pilot was presumed to have been killed. The flight originated about 1250 from Palm Beach International Airport. Controllers said the pilot was flying VFR to Fort Pierce, Fla., and radar services were terminated about 1301. About 10 minutes later the pilot came back up on the approach frequency and advised he was returning to Palm Beach due to an unspecified problem. The controller questioned if the pilot needed assistance and he responded that he had a severe headache at the base of his neck. The controller advised the pilot of a closer airport, but the pilot replied he would stay over the water to avoid the bumps. At 1314, the pilot advised the controller that his defibrillator went off. Approximately 1 minute later, the pilot declared mayday and reported losing his eyesight. He said he was turning east. The controller advised the pilot to proceed to the PBIA, and the pilot responded that he wanted to stay out of their way, and didnt want to take anybody out. Radar data showed the airplane climbing from 1,400 feet at the time of the last transmission to 4,200, then descending to the water at about 2,000 feet per minute. The airplane crashed in an area where water depth is about 500 feet.