Do not always believe your eyes. Illusions exist in nature as well as on the magician's stage. Things that you have already seen or read may affect your imagination and expectations.
In my youth, I spent a month at Katima Mulilo, on the banks of the Zambezi River. It was in the Caprivi Strip area of South West Africa, now called Namibia. One day I was sitting by the river and saw a strange apparition. A large, brown, serpent-like creature was moving through the water. It resembled a vertical sine wave, very like old pictures of monstrous sea serpents or some representations of the Loch Ness Monster, even when viewed through my binoculars.
Local legends do tell of such creatures and at the Kariba Dam further east there is, or was, a statue of a mythological creature that looked very similar to the one that I was watching. It occurred to me that it might be a python but snakes swim like a horizontal sine wave, not a vertical one.
After some time, the "serpent" broke into pieces. It was actually a group of otters playing follow-my-leader! I have little doubt that many legends of lake and river monsters, throughout the world, are based on incidents such as that.