How common are river creature myths in folklores of different cultures?

[Alfred001]

I was reading a collection of short stories by the Japanese writer Akutagawa and one of the stories in the collection is called Kappa, which is apparently a creature from Japanese folklore that lives in rivers and ponds and described as follows by Wikipedia:

They are typically greenish in color^[7] (or yellow-blue^[8]), and either scaly^[9][10] or slimy skinned, with webbed hands and feet, and a turtle-like carapace on their back.

This reminded me of the Creature from the Black Lagoon (aka The Shape of Water creature), although I don't know whether that creature is based on folklore or just an invention of 50s b movie writers, and it also reminded me of a claim I'm often heard that many cultures across the world have dragons in their folklore even though, at the time when their dragon myth originated, these cultures couldn't have had contact.

This got me wondering, could this be true of these Kappa-like river creatures as well? Are river creatures a myth that can be found in many cultures? And do they share these features of appearance that Kappa and the black lagoon creature do?

[Peterkin]

37 minutes ago, Alfred001 said:

Are river creatures a myth that can be found in many cultures?

Yes, they are found on every continent. Stands to reason: people have always depended on rivers; settled near them or travelled on them and fished them. Of course, this also means they sometimes drowned in a river, were carried away by the current; boats capsized, settlements were flooded. Mythological creatures most commonly represent the dangers that the children must be warned against - and what better way than stories? And of course, the creature resemble some aquatic animal - though they often incorporate human features as well. 

[Genady]

Here are some lists:

Category:Piscine and amphibian humanoids - Wikipedia

or more generic

Category:Mythological aquatic creatures - Wikipedia

[wvbig]

It has been determined that there isn't enough food in Loch Ness to support a breeding population of Plesiosaur type creatures, but what if they occasionally enter the lake via a submarine cavern, maybe for breeding/egg laying? I think "Nessi" sightings could be a combination of this and Eel sightings. I saw a video on YouTube of a boat on Loch Ness with what was obviously a large Eel swimming along side of it. I would estimate it to be approximately 10' (3 meters) long

 

[mistermack]

2 hours ago, wvbig said:

It has been determined that there isn't enough food in Loch Ness to support a breeding population of Plesiosaur type creatures, but what if they occasionally enter the lake via a submarine cavern, maybe for breeding/egg laying? I think "Nessi" sightings could be a combination of this and Eel sightings. I saw a video on YouTube of a boat on Loch Ness with what was obviously a large Eel swimming along side of it. I would estimate it to be approximately 10' (3 meters) long

 

No evidence for any of that. There are no 10' eels in Loch Ness. That's almost as unlikely as a Plesiosaur. And if there were, they wouldn't be swimming alongside boats. They are bottom dwelling fish hardly ever seen at the surface.