It's called a mirage, and it's a reflection off a flat surface caused by heat waves. This is a well known phenomenon, and photographers should learn about it to be able to keep their images sharp.
https://fstoppers.com/education/warning-long-lens-shooters-heat-wave-distortion-40508
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Mirage vs reflection
A reflection and a mirage, both superior and inferior, are mirrored images of an object not the actual object itself. The video of the boat is above the horizon line and it is the actual object with a "reflection" of it directly under it. This boat is not a mirrored image of one that is below the horizon line, and actually out of sight, because of the curvature of the earth. If this were a mirage it would be a double mirrored image which would be impossible. A mirrored image of a boat would not have it's mirrored reflected image directly under it. A mirrored image with a mirrored image is never seen.
A reflection is a mirrored image that is upside down and under what is being reflected.
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Here we have a telescopic lens being used to show what is beyond the viewing distance of the viewer which is shown by the line of the sand. The boats and people are actual, they are not mirages. Their reflections are seen directly below them and the extended unseen horizon line is detectable through the boats and their upside down reflection under them.
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Here is a superior mirage above actual ships. The actual ship is not below the horizon line of a curved earth. If that were the case we would have another impossible double mirage, a mirage of the ship and it's mirage above it.
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Here we have an inferior mirage under the object being mirrored. There is no question that the object of the mirage is actual and not a mirage itself. The mirage is right side up and not a true reflection which is upside down of the object it reflects.
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--Dave