The commercial certificated pilot reported that he was landing his amphibian airplane on a lake during a Title 14, CFR Part 91 local flight. He had departed a hard-surfaced runway about 15 minutes prior to landing on the lake. The pilot noted during a telephone conversation with the NTSB investigator-in-charge that as the airplane touched the surface of the lake, it decelerated very quickly and then nosed over. He said he and the sole passenger were able to exit the airplane through their respective side windows. The pilot indicated that there were no preaccident mechanical problems with the airplane, and it was likely that he failed to retract the main landing gear after takeoff from the hard surfaced runway. An FAA inspector who responded to the accident site stated that the airplane had substantial structural damage to the nose section, and that the landing gear was in the down and locked position.