The pilot of the fixed-float equipped helicopter reported he was water-taxiing in a right sideways direction when the helicopter rolled to the right, impacted the water, rolled inverted, and submerged. The pilot, who was the sole occupant, was able to exit the inverted cockpit, inflate his life vest and swim to shore. The impact caused substantial damage to the main rotor system, tail rotor, and fuselage. The pilot reported no preimpact mechanical anomalies with the helicopter. The helicopter's tendency to continue rolling when the critical angle is exceeded, as when one skid is caught on the surface, and the helicopter pivots around that point, is dynamic rollover.