So how does she keep the bottom part from slipping off while she swims?

Looks like her hips are a liiiiittle bit wider than her tail. She's giving us that "come slither" look...

I'm sure she's sick of jokes like that.

Lol, actually I think she might bear jokes like that depending how they're presented and by whom.

She grew up feeling different and isolated from everyone else. So if someone genuinely came up to her with the intent to flirt... First she'd wonder what's wrong with them or what sort of trick they're playing... XD

But then once that's ruled out, she'd probably enjoy it actually. :)

Yeah, I'd say Neovid got half of it right in that that area is a bit wider than the rest of her tail. But most likely, elastic, or like most swimwear, drawstrings cleverly hidden from view. :)

Even if it's slightly wider, that can't be harmless. That swimsuit must slide down (OR UP!) any time she swims.

Sliding down isn't too much of a concern as long as the top portion is tight enough... Up though...

That is a definite danger of naga clothing. I considered sketching some more detailed images to actually fully plan out how underwear and such would work for someone like that, but decided I'd rather not focus too much on that specific feature. I'd stick with elastic and some good fitting as the main factors at work, along with drawstrings to make sure everything is nice and tight.