THE PURPOSE OF THE FLIGHT WAS FOR THE PASSENGER TO MAP THE COAST LINE. PRIOR TO THE FLIGHT, THE PILOT AND PASSENGER AGREED THAT THE FLIGHT WAS TO BE FLOWN AT 400 FT AGL AND AT 75 MPH. DURING FLIGHT, THE PASSENGER MISSED AN AREA AND WANTED THE PILOT TO TURN AROUND. THE PILOT ACKNOWLEDGED AND BEGAN AN APPROXIMATE 15 DEGREE TURN TO THE LEFT TOWARD THE TERRAIN. THE PASSENGER REPORTED THAT AT THE BEGINNING OF THE TURN, THE PILOT DID NOT APPLY POWER. WHILE IN BETWEEN TWO SMALL HILLS, THE PASSENGER FELT LIGHT TURBULENCE THEN SUDDENLY THE RIGHT WING RAISED AND THE NOSE FELL TOWARD THE GROUND. THE PILOT AT THIS POINT APPLIED FULL POWER. THE AIRPLANE CONTINUED IN A LEFT DESCENDING TURN AND COLLIDED INTO THE WATER JUST OFF SHORE. THERE WERE NO MECHANICAL FAILURES OR MALFUNCTIONS NOTED WITH THE AIRPLANE DURING THE POST CRASH INVESTIGATION. DURING THE LAST 90 DAYS THE PILOT LOGGED A TOTAL OF 279 HOURS, ALL IN FLOATPLANES. THREE OF THOSE HOURS WERE IN THE CESSNA 185.