After the pilot landed the amphibious ultralight on a lake, he taxied to the seaplane ramp and parked with about 25 percent of the floats in the water. After about 45 minutes, he attempted a takeoff on the lake and the ultralight would not lift off. He checked the floats and found a significant amount of water inside. He pumped out what he thought was all of the water and attempted another water takeoff; again the aircraft would not become airborne. He began to taxi back to the hangar and decided to perform a takeoff on the grass runway to avoid taxiing over some rough terrain. Shortly after takeoff, the nose pitched up to an excessive angle. The pilot attempted to maintain aircraft control; however the ultralight settled and impacted the runway, resulting in structural damage to the wings and fuselage. The pilot reported that there was water in the floats after the accident and that he did not inspect the floats for water before the final takeoff attempt.