The pilot was attempting to land his float equipped airplane on a lake when he saw a flock of geese near his intended touchdown point. He elected to lengthen his landing approach and fly over the geese. Soon after touchdown, he realized he would not be able to stop the airplane prior to the shoreline, and that he had insufficient room to abort the landing. The airplane ran up on the shore and struck a tree. The pilot reported that he should have performed a go-around.