What happened
The pilot was operating an amphibious aircraft that had taken off from a land airport. The original flight plan involved remaining in the traffic pattern for one additional landing at the land airport. However, while flying on the downwind leg of the pattern, the pilot observed weather conditions deteriorating from the west.
In response to the changing weather, the pilot decided to divert directly to a nearby lake to perform a planned passenger pickup. Prior to this decision, the aircraft's wheels had been extended in preparation for a terrain landing. During the approach to the water, the pilot failed to retract the landing gear. As the aircraft touched down on the water, the extended wheels caused the aircraft to flip inverted.