The pilot reported that he and his wife were sightseeing along the Yellowstone River just south of Laurel, Montana, when he spotted what he described as unusual animal tracks on a gravel sand bar adjacent to the river. He applied one notch of flaps and executed a 180 degree course reversal for a "low and slow look." The aircraft subsequently struck the water and nosed over. The pilot reported that there was no mechanical malfunction or failure with the aircraft or its powerplant/systems.