THE PLT STATED THAT HE PERFORMED HIS RUN UP CHECK WITH 'NORMAL RESULTS.' TEN DEGREES OF FLAPS WERE SET FOR A SOFT FIELD TAKEOFF, & DURING THE TAKEOFF ROLL, 'THE AIRCRAFT RAN NORMAL.' AT APRX 55 MPH 'THE AIRCRAFT LEFT THE GROUND AND IMMEDIATELY IT SETTLED DOWN & FINALLY GOT AIRBORNE'. HE THEN MAINTAINED 55 TO 60 MPH & AT APPROX 150 FT. 'THE AIRCRAFT DID NOT WANT TO CLIMB ANY FURTHER'. HE THEN STATED THAT THE THROTTLE, MIXTURE AND CARBURETOR HEAT WERE FULL IN AND HE LOOKED AT THE ENGINE GAUGES AND THE NEEDLE WAS 2000 TO 2100 RPM, 'WITH SLIGHT FLUCTUATIONS BETWEEN THESE NUMBERS', HE THEN CHECKED THE MAGS, BOTH TO LEFT AND RIGHT BUT NO IMPROVEMENT WAS OBTAINED SO HE SWITCHED IT BACK TO BOTH. HE THEN HELD AN AIRSPEED OF APPROXIMATELY 60 MPH AND MANEUVERED THE AIRCRAFT TO TRY TO DITCH IT IN THE LAGOON. NO DETERMINATION COULD BE MADE AS TO THE NATURE OF THE ENGINE MALFUNCTION AS DESCRIBED BY THE PILOT.