Quote from NTSB document:
The National Transportation Safety Board has determined today that popular entertainer John Denver fatally crashed his experimental aircraft into Monterey Bay because his attention during flight was diverted in an attempt to switch fuel tanks. The fuel selector valve on the amateur-built Adrian Davis Long-EZ airplane Denver was flying was behind the pilot's left shoulder, forcing him to turn in his seat to locate the handle. This action, the Board concluded, likely caused him to inadvertently apply the right rudder, resulting in loss of aircraft control.