The pilot/owner and the pilot-rated passenger departed in the amateur-built amphibious Mallard II from the land-based airport, in order to practice water landings on a nearby lake. The purpose of the flight was to provide the passenger with experience that would assist him in making a decision about purchasing a similar airplane. The passenger was seated in the left seat, and the pilot was seated in the right seat of the dual-control airplane. The pilot reported that the airplane's tail lifted excessively when takeoff power was applied, and that this, combined with an unrelated problem with the headsets, distracted the two occupants during the 3-minute flight to the lake. As the airplane approached for the first landing on the lake, the pilot discussed the descent, power settings, wind conditions, and airspeeds with the passenger. However, neither the pilot, nor the passenger who was manipulating the controls, utilized any pre-landing or landing checklists, and they neglected to retract the landing gear for the water landing. When the landing gear contacted the lake surface, the airplane nosed over inverted, which substantially damaged the fuselage and wing.