THE ACFT STRUCK A GRAVEL BAR WHILE TAXIING. THE ACFT WAS NOT FREED FROM THE GRAVEL BAR UNTIL THE NEXT MORNING DUE TO A RECEDING TIDE ON THE PREVIOUS NIGHT. APPROX 3 HOURS LATER AS THE PILOT TAXIED AROUND AWAITING DAYLIGHT FOR TAKEOFF, THE ACFT ROLLED OVER IN THE WATER AND EVENTUALLY SANK. PRIOR TO THE DISAPPEARANCE OF THE ACFT, THE PILOT NOTICED TWO 1 INCH DENTS IN THE SUBMERGED FLOAT WITH BUBBLES COMING OUT OF THEM. THE PILOT ALSO STATED THAT THE FLOATS ROUTINELY LEAKED AND NEEDED TO BE PUMPED OUT DAILY, HOWEVER HE INDICATED THAT THIS WAS NOT ACCOMPLISHED BETWEEN THE TIME THE ACFT STRUCK THE GRAVEL BAR AND THE TIME IT SANK. THERE WERE NO APPROVED PERSONAL FLOATATION DEVICES ABOARD THE ACFT. THE FOUR INDIVIDUALS ABOARD THE PLANE USED INFLATED HIP WADERS DURING AN ATTEMPT TO SWIM FOR SHORE. TWO INDIVIDUALS WERE LOST AND PRESUMED DROWNED IN THIS ATTEMPT. WATER TEMPERATURE WAS APPROX 45 DEGREES F.