Well, the thing about the sky - its main distinguishing feature - is it's blue. And the thing about the sea, the colour of sea, your basic sea colour, is blue. So...
Unfortunately, this seemingly simple observation was apparently lost as David Morris’s photo of a large vessel seemingly floating in the sky off of the Cornish coast found itself the subject of both churnalism and misguided scepticism worthy of the worst of the flat-Earth community.
A perfectly reasonable explanation for the appearance of the vessel in the image above had been posited in the original reporting of it by Chris Matthews of Cornwall Live:
“It is likely the remarkable optical illusion was caused by a cloud formation closer to the shore, which changed the colour of the water closer to the land. The boat, being further away, was in a cloudless area and therefore the sky reflected the sea - making it look like the boat was floating.”
And early churning from ITV and the Daily Express retained this explanation. However, when the BBC then churned the image they attempted to justify their action by adding the standard explanation for such photos of superior mirage courtesy of meteorologist David Braine, and this was the explanation that was attached to subsequent re-churnings by the likes of The Guardian.
While, a little late to the party, the Daily Mail further compounded the confusion by churning the image alongside an entirely predictable quote from an unnamed spokesman for the Met Office conjuring up the more complex but far cooler sounding fata morgana mirage as an explanation.
Mick West of Metabunk has done a convincing job of explaining why a false horizon caused by weather conditions, as the witnesses themselves, who have a clearer and more complete view of the situation, often state, is a more likely explanation than the mirage of these experts' summary dismissals.
Nonetheless, with this possibly erroneous explanation attached, some in the skeptic community seized upon the image as our own apparent black swan, challenging flat-Earthers to explain it or abandon their patently false beliefs, apparently oblivious to how badly this has turned out when the roles were reversed.
Even worse were those so-called skeptics who just completely ignored all these explanations and decried the whole thing as CGI Photoshop in a conclusive leap which even for flat-Earthers has become something of a tired trope.
The image is, however, apparently real and can be just as easily explained using phenomena that would probably work as well on a flat-Earth as they actually do on an irregularly shaped ellipsoid, and all that is required to discover this is to go back to the original source.