Glassy water, or unbroken snow operations are a skill unto themselves. Glassy water landings are a requirement for a float rating - I was training them to a fellow yesterday. I will state that if you're not planning to land on it, you should not be within 150 feet of glassy water or unbroken snow by barometric altimeter (and radar altimeter is not a lot better). A glassy water landing is specifically set up have a rate of descent to the surface of 100-200 FPM, to contact, with no flare. Challenging in a floatplane, easier in a flying boat. But there is no other safe way, if the surface is your only available reference. Otherwise, you get tight into shore, for a reference.
The risk for those flying low over the water, without the intent to land on it, is that on those wonderful calm opportunities, which just draw you into flying that time for the sheer delight, you can get down [too] low to water with a light ripple, just perfect for reference, to have it instantly calm to glass when the light breeze stops, and you 'got nothin' for altitude reference. By the time you get back on to the altimeter ('cause you were not eyes in), and figure out what altitude would be safe, you've plowed in.
These are common accidents in floatplanes, and the training to recognize and avoid is thorough for float or ski flying. Unfortunately, wheelplane pilots don't get that training. Unbroken snow can be worse, because it can change altititude (elevation) under you. Elevation changes do occur on bodies of water, but it's very calm there!