Another classic situation where it shows that if you want to be as safe a pilot as possible, you should never stop reading up on the activity. Accident reports, safety stories in magazines, etc.
We can't experience it all but we can get as much experience as possible by learning as much as possible. It is no guarantee but it definitely improves the odds. Sadly, many don't take this risky hobby seriously enough and the carnage that we see in general aviation is the result.
As mentioned above, floatplane fliers have likely done specific training for this but as a regular landplane pilot, you may not be familiar with thie phenomenon of glassy water or forgotten about it as it was a brief paragraph from private pilot training.
If you are going to fly low over the water, you will lose depth perception even on a beautiful day over smooth water. If you see reflections of clouds in it, for example, that is a good hint. Stay clear.
This accident was totally unnecessary. But it wouldn't surprise me if the pilot was unfamiliar with this phenomenon.
Read up and learn from others. Flying has endless dangers.