One problem nudists face in OpenSim is that it's often unclear whether or not a sim allows public nudity. This is because you can't go by the rating. Even an Adult rating does not automatically mean that public nudity is allowed.
In Second Life, it is (or used to be, I'm not sure, I'm not in Second Life) clear-cut: Moderate means non-sexual public nudity is allowed by default unless the owner of an area says otherwise on in-world signs, and AFAIK, even then, nudity in private is still allowed. Adult means that not only public nudity but also public sexuality are allowed. Full stop. No "unless". You don't want nudes in your area, then go Moderate and put up signs, or go General right away, otherwise Linden Lab might make you.
In OpenSim, first of all, grids don't have identical rules as far as ratings are concerned. Either the rules differ from grid to grid, and even then, no grid has taken over Second Life's rules, or grids (the vast majority actually) don't have any written rules for the ratings. Even if there are rules, these are usually flexible for sim owners. Adult gets its "unless"; sim owners may limit it as far as they want to. Moderate, in turn, is hardly distinguishable from General in practice, maybe just by whether or not a bar that puts alcoholic beverages on display and/or implies the selling of alcoholic beverages can be installed or something. But as far as liberties for the avatars go, General and Moderate are usually exactly the same. That is, maybe you mustn't use swear words on the public chat in General-rated places.
For one, this leads to rampant misuse of the Adult rating. There is no limit on how far you can stretch it and make it stricter. So what some sim owners do is take all avatar liberties away that usually come with the Adult rating. All of them. Everywhere on the sim. You get sims with the liberties of the General rating and the limitations of the Adult rating. The latter used to mean, "Beware of nudity and/or sexual content anywhere and everywhere in this place," but it's bent to only mean, "Kids stay out!"
The reason behind this is because older, conservative sim owners want to make places where grown-ups can be amongst their own. They "don't want no kiddies raisin' hell" in their blues clubs which are perfectly child-friendly otherwise (save for the Jackie bottles at the bar, maybe), so they use the Adult rating to keep child avatars out. Never mind that behind every last child avatar in OpenSim is an adult real-life person, and what few kids actually use OpenSim have exaggerated, oversized, decidedly badass adult avatars that can walk around BDSM places as they please. (Remember your own childhood? How often have you, as a kid, ever role-played another kid? See?)
But there's more to this: Many owners of Adult-rated sims can't be bothered to provide written rules for their sims, not even if they differ from established standards. This is not only laziness or carelessness. No, they could never imagine that this may be necessary because they believe everyone else thinks like them, and their idea of Adult is the one Hypergrid-wide established standard.
So on the one hand, there are people who limit the Adult rating on their sims to General liberties without telling anyone. "I don't have to put up signs or write rules. Everyone knows that nudity is banned in public!" U.S. citizens will wonder how one could ever question this. Citizens of Denmark will say this is factually wrong, as will Second Life users.
On the other hand, there are people who employ the very same rules as Second Life for the Adult rating on their sims, again, without telling anyone. "I don't have to put up signs or write rules. Everyone knows that public nudity is allowed in Second Life and OpenSim!" First of all, the "OpenSim" part is dangerously wrong as you've just read. The "Second Life" part is correct. But people who have encountered Adult-rated sims with only General liberties will be too cautious to use these places to their full extent.
On the one hand, you have nudists being kicked and banned on the spot for public indecency for going nude on an Adult-rated sim. Nobody has told them that "Adult" only means "no kiddies allowed" because the sim owner thinks that's an established standard that doesn't need to be communicated. (By the way, such incidents have led to formerly Adult-rated sims being forcibly downrated to Moderate because exactly nothing on them justified the Adult rating. Some sim owners try to circumvent this by providing exactly one well-enclosed and often well-hidden nook with one piece of sex furniture to justify the Adult rating while banning nudity everywhere else.)
On the other hand, you have Adult-rated sims that technically do allow or even encourage nudity, but to the sim owners' dismay, everyone stays fully clothed because it's written nowhere that nudity is allowed or encouraged. I think some are even afraid of wearing swimwear on beaches and keep their streetwear on because they think a bikini or a pair of swim trunks worn topless may already break some unwritten rules because it shows too much skin. So you may end up as the one sole nudist amongst fully dressed people who either ask you why you're naked or even demand you put some clothes on.
There is one more contributing factor to this uncertainty: The rating on OpenSimWorld is not always the same as the in-world rating. Not exactly few sims are advertised as Adult-rated on OpenSim, but they're Moderate-rated or even General-rated in-world. And when you're hypergridding, you can't read a sim's in-world rating on the map before teleporting there.
So if you're a dedicated nudist who goes in the nude wherever it isn't explicitly forbidden, you may discover an Adult-rated sim on OSW and teleport there in the buff as you always do, just to find yourself naked on an actually General-rated sim. If you don't care if someone is on the sim when you teleport in, you may even end up naked in front of the sim owner who will promptly ban and block you before you can explain that they've falsely advertised their sim as Adult-rated.
Two things would make nudists' lives much easier. For one, at least those sims that don't grant Adult liberties with an Adult rating (= that have an Adult rating but ban nudity) should say so in public, not only in-world, but also on OSW. Besides, sim owners should check and double-check if the ratings for their sims in-world and on OSW are identical.