The pilot lost control of the amphibian airplane during a 'high speed step-taxi' after landing. The pilot noticed a swell left by a boat that had departed the landing area and elected to approach the swell at a 45 degree angle. As the airplane crossed the swell, the pilot felt the right float 'dig in.' In an effort to prevent the floats from sinking further, the pilot pulled full aft on the yoke and applied full power. The airplane nosed over inverted and began to sink.