Yes, there is a basis for it. In Herodotus, Book II:
 Anyone, Egyptian or foreigner, known to have been carried off by a crocodile or drowned by the river itself, must by all means be embalmed and wrapped as attractively as possible and buried in a sacred coffin by the people of the place where he is cast ashore;
 none of his relatives or friends may touch him, but his body is considered something more than human, and is handled and buried by the priests of the Nile themselves.
Aaron Cheak, of the University of Philosophical Research writes more on the topic in an article titled: "Waters Animating and Annihilating: Apotheosis by Drowning in the Greek Magical Papyri" (Yes, sketchy-sounding name aside, the Greek Magical Papyri are a real thing). He asserts that this apotheosis by drowning was due to the connection to the death and ressurection of Osiris:
Only later, from the New Kingdom through to the late period, were the drowned increasingly regarded as sacred. This is due chiefly to the increasing significance of the role of Osiris, for whom drowning becomes instrumental to his revivification and deification. As Hopfner observes: “only after death by drowning could he [Osiris] become a god.” It is thus the myth of Osiris—and more significantly his death and resurrection at the hands of his enemy, Seth-Typhon—that underpins the motif of apotheosis by drowning that we meet in the magical papyri.
Also worth mentioning, there is one significant example of a drowning in the Nile leading to a good and proper Roman state deification, complete with widespread cult. Antinous, a lover of Emperor Hadrian, died by downing in the Nile. Emperor Hadrian was devasted, and, in an unprecented move (for someone not in the ruling family), Hadrian deified him. His cult spread to at least 70 cities, and at least 115 scultures of him are known to survive today.
The first blog linked in your question seems to have conflated these two somewhat, although I didn't really give it a thorough read.