The water-bound serpent/fish teacher who takes nothing and leaves everything: both figures deliver a complete civilizational curriculum (writing, law, temple, calendar/geometry), consume nothing while doing it (Oannes 'took no food'; Quetzalcoatl's treasures are left behind), and then exit across or into the sea — one nightly, one finally — with the record explicitly closed at their departure: 'nothing material has been added' since Oannes; 'no one knows how' Quetzalcoatl arrived where he was going. In both, the sea is where the teacher is native and where the account is permitted to end.
Oannes, body of a fish with a man's head and feet, rises daily from the Erythraean sea; he 'was accustomed to pass the day among men; but took no food'; he teaches letters, sciences, city-building, temples, laws, geometry — and 'when the sun had set... retired again into the sea, and passed the night in the deep; for he was amphibious. From that time, nothing material has been added by way of improvement to his instructions.'
The plumed-serpent teacher of the arts, leaving Tollan, reaches the coast: he 'constructed a raft of serpents, and seating himself on it as in a canoe, he moved out to sea. No one knows how or in what manner he reached Tlapallan.'
Both passages verified verbatim in the cited public-domain editions; the no-food detail and the raft-of-serpents detail are in the texts' own words.
Our fact-checkers corrected the first draft:
The same three-stage avian reconnaissance protocol from a grounded vessel, with overlapping species (dove, raven) and the identical inferential logic: the bird that does not come back is the good news.
An identical four-beat sequence: landing → burnt offering → the deity smells the smoke and is moved → vow of never-again sealed by a physical token of remembrance (a string of sky-blue lapis at the goddess's throat; a bow of color in the cloud).
In both, heaven never announces the flood openly.
Same skeleton with no shared geography: humanity's crime is an affront to the senior god's comfort or dignity, a council formalizes extermination, the killing is halted mid-course by subversion within the pantheon itself, a remnant survives, and the wearied god afterward binds or removes himself.