This claim emerged from scholars reasoning backward from a pre-determined conclusion. In those days, it was widely accepted that the Biblical account of creation and the Flood was literally true. Since it was literally true, it must therefore be the case that all other religious beliefs were false. To the degree that they were similar to the Bible it could only be due to the pagans corrupting the Biblical truth.
Thus, when scholars like Jacob Bryant read the fragments of Berossus, the Babylonian priest who described the Great Flood in detail similar to that of the Bible, it proved that the Babylonians had recorded a corrupt tradition of Noah’s Flood. From there, it was a short hop to identifying Jason and the Argonauts as Noah and the Ark (for Argo = Ark, they thought, assuming English to be a universal language), and seeing the Babylonian fish-god Oannes as Noah himself, Oannes being a corrupt form of Noah’s name.
This belief was taken to absurd levels of spurious detail, seeing in every random word syllables related to “Noah” and the “Ark” (in English, of course) and in every boat or floating container the ship of Noah. Henry Lee outlined the theory, which he took very seriously, in his Sea Fables Explained in 1883, and it is laughable reading today. He first outlines the story of Oannes, half-fish and half-man, who rose up from the sea to teach the Babylonians civilization:
In this tale we have a distorted account of the life and occupation of Noah after his escape from the deluge which destroyed his home and drowned his neighbours. Oannes was one of the names under which he was worshipped in Chaldea, at Erech ("the place of the ark"), as the sacred and intelligent fish-god, the teacher of mankind, the god of science and knowledge. There he was also called Oes, Hoa, Ea, Ana, Anu, Aun, and Oan. Noah was worshipped, also, in Syria and Mesopotamia, and in Egypt, at "populous No," or Thebes—so named from "Theba," "the ark."
The history of the coffin of Osiris is another version of Noah's ark, and the period during which that Egyptian divinity is said to have been shut up in it, after it was set afloat upon the waters, was precisely the same as that during which Noah remained in the ark.
Dagon, also—sometimes called Odacon—the great fish-god of the Philistines and Babylonians, was another phase of Oannes. "Dag," in Hebrew, signifies "a male fish," and "Aun" and "Oan" were two of the names of Noah. "Dag-aun" or "Dag-oan" therefore means "the fish Noah."
From a few scraps of dubious philology, false analogies, and assumed conclusions an elaborate mythology of universal Noah worship was created. There is precious little difference between the processes that yielded this theory and those at work in the ancient astronaut theory.