A NAVIGABLE WATERWAY, HARVEY CANAL, WAS BEING USED AS A SEAPLANE BASE. IN THAT AREA, A SET OF POWER LINES WERE SUSPENDED OVER THE CANAL BY 2 TALL TOWERS. THE LOWEST LINE OVER THE CANAL WAS 160 FT ABOVE THE CANAL SURFACE, BUT THE LINES DROPPED TO HEIGHTS OF DOWN TO 60 FT OVER LAND. THE NORMAL PROCEDURE FOR A SOUTH TAKEOFF WAS TO FLY UNDER THE LINESAFTER LIFT-OFF OR TO TURN & CROSS OVER THE LINES WHERE THEY ARE LOW TO THE GROUND. WHILE TAKING OFF, THE ACFT WAS OBSERVED TO CLIMB NORMALLY TO APRX 30 FT AGL, THEN BEGAN TO ACCELERATE. APRX 500 FT NORTH OF THE POWER LINES, THE ACFT ENTERED AN ABRUPT CLIMB & REPORTEDLY WENT TO A NEAR VERTICAL ATTITUDE. IT THEN ROTATED ABOUT THE LEFT WING & DESCENDED VERTICALLY INTO THE CANAL, IMPACTING APRX 100 FT NORTH OF THE POWER LINES. AN EXAM REVEALED NO EVIDENCE OF A PREIMPACT MALFUNCTION OR FAILURE. AN INVESTIGATION REVEALED THAT THE PLT HAD BEEN GIVING FLT INSTRUCTION TO THE PASSENGER. IT WAS NOT VERIFIED WHO HAD PULLED THE ACFT UP, BUT THE PLT'S NORMAL PROCEDURE WAS TO FLY UNDER THE WIRES.