It is on basis of the Sinking Ship Effect that the ancients declared the earth to be a globe.
Not quite. The "Sinking Ship Effect" was just one of the pieces of evidence.
Pythagoras' pupils, if not the great man himself, knew that the Earth is round. Traveller's tales of ships disappearing over the horizon and the Pole Star shifting to a higher position in the sky as one journeyed north suggested a curved Earth.
sometime between 500 B.C. and 430 B.C., a fellow called Anaxagoras determined the true cause of solar and lunar eclipses - and then the shape of the Earth's shadow on the Moon during a lunar eclipse was also used as evidence that the Earth was round.
Around 350 BC, the great Aristotle declared that the Earth was a sphere (based on observations he made about which constellations you could see in the sky as you travelled further and further away from the equator).
And during the Greek period there were measure of the:
distance to the moon (not vastly different from the modern value),
distance to the sun (vastly less than the modern value, but still some 9 million kilometers away) and
the circumference of the earth (probably close to the modern value but doubts remain.
This was just the work of the early Greeks but that was extended and more accurate measurements done by the early Arabs, Persians and Indians in the latter half of the first millennium till around 1200 AD.
Some might think it strange that the observation that the sun (moon and stars) appear to rise from behind the horizon and to set behind the horizon was not one of these pieces of evidence.
But that was never an issue. The earlier flat-earth "models" of the Babylonians and earlier Greeks already included that as something quite obvious.
However, the inconsistency of the Sinking Ship effect is more evidence against the Round Earth Theory than it is for it. The Sinking Ship Effect is supposed to prove that the earth is a globe, but it is often inconsistent.
I totally disagree. The variability of atmospheric conditions has been known and investigated for a long time.
A light path just skimming the water (or land) surface often produces anomalous propagation when the water (or land) surface temperature differs greatly from the air temperature.
More strikingly, it has been seen in previous threads that the Sinking Ship Effect does not reflect the Round Earth prediction for how much should be hidden.
As noted above, such variability is to be expected. To be meaningful these observations must be repeated under different conditions and in different seasons.
In the experiment in the OP the flash of light appears exactly at the water line, no higher and no lower.
As I said above, "exactly at the water line, no higher and no lower" is precisely where the temperature gradient is highest.
If the water differs greatly from the air temperature some sort of anomalous propagation.
Bit so often flat-earthers quibble about the hidden height not matching the Globe expectations when if the earth were flat none should be hidden.