5. In Greek myth the 'sudden plunge' of Sun probably was the background against which another version was spun, that about Phaeton, the reckless youth who once drove Sun's chariot:

"In the story of Phaëton, which is another name for Helius himself (Homer, Iliad xi. 735 and Odyssey v. 479), an instructive fable has been grafted on the chariot allegory, the moral being that fathers should not spoil their sons by listening to female advice. This fable, however, is not quite so simple as it seems: it has a mythic importance in its reference to the annual sacrifice of a royal prince, on the one day reckoned as belonging to the terrestrial, but not to the sidereal year, namely that which followed the shortest day.

The sacred king pretended to die at sunset; the boy interrex was at once invested with his titles, dignities, and sacred implements, married to the queen, and killed twenty-four hours later: in Thrace, torn to pieces by women disguised as horses ... but at Corinth, and elsewhere, dragged at the tail of a sun-chariot drawn by maddened horses, until he was crushed to death. Thereupon the old king reappeared from the tomb where he had been hiding ... as the boy's successor ..." (Robert Graves The Greek Myths.)

At the end of the 'domain of water' another gap had to be bridged in order to reach the 'dry land of life' again.

Phaeton, a feeble son of Sun, could not control his father's horses - he was too weak.

"... And there is little doubt, in fact none, that Phaethon (in the strange transformation scenes of successive ages) came to be understood as Saturn. There is the testimony of Erastosthenes' Catasterisms, according to which the planet Saturn was Phaethon who fell from the chariot into Eridanus, and Stephanus of Byzantium calls Phaethon a Titan ..." (Hamlet's Mill)

If he could not reach 'dry land', then he instead must fall back into the 'land of water'.