The pilot of the float equipped airplane reported that he intended to practice water landings and operations after about 7 months of seaplane inactivity due to winter. He was flying above a lake with glassy water conditions surveying the surface for ice or other hazards when the airplane's floats inadvertently contacted the water "a bit harder" then normal. The pilot elected to reduce engine power and attempted to land; however, the airplane nosed over into the water, and subsequently sank. Postaccident examination of the airplane did not reveal any abnormalities that would have precluded normal operation. The right float displayed evidence of contact with the propeller and the aft portion of the fuselage, near the empennage was substantially damaged. The pilot reported about 1,400 hours of total flight experience; which included about 50 hours in the same make and model as the accident airplane. In addition, he reported 21 landings on water during the 12 months that preceded the accident, with the most recent water landing about 7 months prior.