So some people had the argument that this is based on fish and mermaids could be considered mammals, which is true. So let's compare to mammals living in the ocean:
This is a dolphin skeleton:
As you can see, no knees. The way they swim is also similar to that of a mermaid.
Some have also pointed out that there would be no need for a pelvis, but as you can see, these two still both have them, though barely.
If we go with evolution, mermaids are likely way younger than either of these two, so they didn't have the time to de-evolve the pelvis yet. Maybe in 10 more million years, mermaids would also start lacking proper pelvic structures. Or maybe it would still be there, if the reproductive organ is still where it would be on humans.
I've also seen the beluga whale argument. This is how their skeleton looks like:
They don't have knees either. That picture that so many are sharing isn't a knee, it's fat and skin. Idk where the belief came from that they have knees, but they certainly don't.
Now the seals argument. Yes, they do have knees, but that is part of their hind limbs. Not to mention when they swim, it doesn't bend like that either, it still looks flowy. The equivalent of that would be if we put knees on the end of a mermaid tail, before the caudal fin starts, as that is usually pretty bendable, like it is with fish. Here is a picture of a seal skeleton:
So this could be the fin that has some leftover structure of legs, while the rest of the mermaid tail could be an elongated spine. Though if that was the case, the tails would most likely be shorter.
What's incorrect with comparing seals though, is that they are semi aquatic. Mermaids live in the ocean and are fully aquatic. Whether they can breathe underwater or if they need to come up for air like dolphins or whales is something different that fully depends on how the person wants them to be, but they're still fully aquatic, so the two don't compare. However they could still have a leg like structure as the caudal fin, which I will draw myself and post it here later.
And while I agree with people that this is a mythical creature that doesn't exist, we still need to take into account that you can't draw something and disregard biology entirely, even if that is an art style. Say manga style for example. The proportions still need to be realistic, even though the art style is unrealistic. You can draw dragons, but you can't just make them have bodies that contort in ways that's impossible. Yes, they aren't real. And yes, you can draw a mermaid as a part eel, or snake, or octopus or whatever else, but even then you have to take into account how that animal moves and is, and add that to the human movements. Now, does this mean you can't bend a mermaid tail? Absolutely not. You can! Make it really flexible, so you can bend it, but make the bend softer. For that, I think looking at snakes is actually perfect. Look at how their bodies look when fully bent. That's how they should be. But a mermaid having a knee, especially where it is usually depicted, is just entirely unscientific. Yes I know science isn't the most important part of drawing, but when you want to make it look nice, you have to add that to the mix as well. Same with drawing anything else, really. People will see if something is off about a drawing of a face, of a dog, of a butterfly, of a tree. There are only certain ways you can twist something into an art style before it becomes completely incorrect. One show and its spinoff that managed to make this look well (it's the only one I'm familiar with, so there may be more), is H2O and Mako Mermaids. The actors only have fake tails on, but still they managed to swim in a way that makes their tails look realistic, there are some scenes where they do bend their knees a little too much and it's visible if you're looking, but majority of the time they swim in a way that makes their bodies look fluid. Those two series I think are perfect for "real" mermaid references, with how they swim and move in the water. Especially because their tails and swimming in general wasn't all CGI, but actual molds and suits, and they had to take lessons and the swimming scenes were in pools, the beginning of the ocean or most times in the aquarium in Australia that I forgot the name of. So they were actually moving in water.
So anyway I will post later on (hopefully today) how a mermaid with seal-like bone structure would look like. I already know it won't look pleasant...as I already have something pictured in my head. I hope I'm not pissing people off with this lol.
Also, to the "look at how snakes bend" argument, here are some pictures, that you can reference when drawing mer people with bent tails: