PLT OF A FLOAT PLANE CAME CLOSE TO BOATS AND A BRIDGE DURING INITIAL CLB DURING TKOF FROM A CONGESTED RIVER WITHIN A LARGE CITY.

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I WAS DEPARTING THE WILLAMETTE RIVER IN DOWNTOWN PORTLAND OREGON; AS THE SOLE OCCUPANT OF THE ACFT. THE WINDS WERE OUT OF THE N AND ABOUT 10 KTS; BUT I COULDN'T DEPART NBOUND BECAUSE OF NUMEROUS BRIDGES XING THE RIVER. IT WAS ALSO A HOT SATURDAY AFTERNOON WITH THE TEMP AROUND 80 DEGS F AND NUMEROUS BOATS WERE PRESENT ON THE RIVER. WHEN I HAD LANDED; I OBSERVED THAT MY TKOF WOULD OCCUR BTWN 2 BRIDGES; SO I INSPECTED THEM BOTH FOR WIRES OR OBSTRUCTIONS. FINDING NONE; I DETERMINED THAT THE TKOF AND CLBOUT COULD BE ACCOMPLISHED BTWN THE 2 BRIDGES. DURING THE TKOF RUN; I WAS FORCED TO MANEUVER BTWN SEVERAL BOATS. IN ONE INSTANCE; AS I TURNED R TO MANEUVER AWAY FROM A BOAT; THE BOAT TURNED L AND DIDN'T FOLLOW THE RULES OF THE WATER. I AVOIDED THIS BOAT AND PERFORMED A SHORT FIELD TKOF TO AVOID OTHER TFC. AT THIS POINT; I REALIZED THAT IT WOULD BE UNSAFE TO TRY AND CLBOUT OVER THE BRIDGE OR SHORELINE; SO I FLEW UNDER THE BRIDGE WHICH IS HIGHER THAN 125 FT OF CLRNC. I THEN PERFORMED A NORMAL CLBOUT TO CRUISE FLT. I BELIEVE THAT THIS SIT OCCURRED BECAUSE OF A COMBINATION OF TAKING OFF DOWNWIND TO AVOID THE NUMEROUS BRIDGES UPWIND. THE HOT WX; AND THE NUMEROUS BOAT TFC THAT WAS PRESENT ON A SATURDAY AFTERNOON ON THE RIVER. TO PREVENT THIS FROM HAPPENING AGAIN; I WOULD SUGGEST NOT USING THIS AREA FOR SEAPLANE OPS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THE FAA INVESTIGATED THIS INCIDENT AFTER A GND OBSERVER COMPLAINED ABOUT HIS ACFT COMING CLOSE TO BOATS. HE WAS OPERATING A DEHAVILAND BEAVER ON STRAIGHT FLOATS IN AN AREA CONGESTED WITH BOATS. AT LEAST 2 OF THE BOATS DID NOT YIELD TO HIS TKOF FLT PATH AND 'CUT' IN FRONT OF HIM MAKING IT NECESSARY FOR HIM TO TKOF PREMATURELY. THIS RESULTED IN INSUFFICIENT ALT TO GO OVER A BRIDGE DURING INITIAL CLBOUT. THE FAA INSPECTOR DECIDED TO TAKE NO ACTION AND THE MATTER WAS DROPPED.