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Getura Merrow are totally different creatures from nagas. They are essentially the Scottish and Irish verson of merfolk... And actually the creature I have been trying to remember the name of for the past few days! (I wanted to make a joke that the characters from Free! may have been a result of a cap-less Merrow and a human)
As for the more traditional Naga, it literally depends on what culture you are looking at. The epic
Mahabharata, for example has a mishmash of Nagas that are humans with snake traits and snakes with human traits and shape-shifting snakes but (at least according to my resources which at this point is mostly wikipeda, google is being unhelpful) no association with water. In Hinduism on the other hand, they have a heavy association with water, and are often guardians of bodies of water. Past that there is a lot of culture-blending where the naga become synonymous with other serpents and dragons of the region. Usually multi-headed dragons that can shape-shift, treasure-guarding deities, and beaked water serpents.
So as you can see, it really all does depend on what the author of said fantasy thing is going for. Also, maybe it is just because I've never played any of the games you listed (though I have heard of them and I do have a handful of MtG cards because I love the artwork) but I, as well as many people I know, when they think of nagas they think of land-based upper-half human lower-half snake people. Not saying that the aquatic naga isn't well-known, but I do believe that the land naga is more popular.
The Lamia that is associated with snakes is the Greek one and is far more prevalent. And also has many interpretations and forms (yet another poor woman cursed by Hera, Zeus needs to learn to keep it in his pants), and he was like a boogeyman of the time. A story to tell children to scare them into doing good.
The duck-footed one mentioned earlier seems to be from Basque mythology, which was (apparently) quickly replaced by Christianity in that region.
Both Lamia appear to be female-only, while Naga can be both male and female. The females are sometimes called Nagini, sound familiar HP fans?
tl;dr: It doesn't freaking matter because Naga and similar creatures were found all over Asia and it's multitudes of belief systems and cultures and some naga have absolutely zero human traits at all! It's a mish-mash!
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Rhea
Perhaps? Though, while that person's intentions are good... They apparently haven't been around fantasy genre anything for the past few years (or any art website with a significant number of anthro artists, furry fandom members, and lovers of monsterboys/girls). Naga have become so frequent and prevalent in all kinds of colors (both normal human skintones and not-so-normal skintones) it is kind of a stretch to put blame of white-washing on FR.
Is it a golden opportunity for throwing in some racial variation.... But media (mostly the internet) is so saturated with white naga now-a-days it seems more like a missed opportunity than actual white-washing.
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Pisces
I've noticed Lamia has come to mean snake-lady like how Pegasus has come to mean winged horse (remember: in Greek mythology Pegasus was the name of a specific horse). I dunno why I guess it's just a thing people do. Mostly in video games and stuff.