The pilot reported that he was practicing landings on a lake in his experimental amateur-built amphibious airplane. During the accident landing, he reported that the airplane "skipped slightly about 2 feet off the water, I held attitude and let it come down again but I accidentally let the left wing tip sponson settle onto the water while still going at about 55 knots." The airplane started to turn to the left, the sponson dug in more, and he reported that he did not correct the situation quickly enough. The airplane turned 180 degrees before coming to rest. Once the airplane was towed to shore, substantial damage was noticed to both wing sponsons and the rudder. The pilot reported there were no pre-impact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.