While on a heading of approx 270, the pilot was instructed to follow another airplane into the airport. He did not turn in on final and continued for almost 3 miles. When the ATCT told him to turn to 180 for resequencing, he turned to 330, toward the airport and started a descent from 2000 feet. The airplane descended to 1600 feet, and then started a climb back to 2000 feet, with a shallow left turn, contacted approach control and continued the turn to 150. The radar data showed the aircraft speed started to decrease when the airplane climbed. There was a continued decrease of speed to 44 knots, at which point, the airplane started a descent, followed by a left spin, and impacted water. The radar data showed no rapid course changes prior to the low point of speed. The air/ground tapes showed the pilot sometimes took almost a minute between frequency changes, and his voice was indecisive and hesitant. The pilot had been treated for seizures since 1974. Anti-seizure medication was found in his toxicological samples.