Mark Haskell Smith’s worst nightmare goes like this. He’s in the company of 2,000 strangers when he suddenly realises that he is the only one wearing clothes. Panicking slightly at all those sagging nads and pendulous breasts (everyone except him appears to be over 70), he decides that he has no option but to “drop trou” and join in. This, by rights, should be the moment when Haskell Smith wakes up sweat-drenched. Instead, he finds himself in the Ocean Bar, sipping a beer butt naked. This, it transpires, is not a dream but a working day. Haskell Smith is on board the Big Nude Boat to investigate, undercover, the state of contemporary naturism.

It turns out to be a declining world, or at least a splintering one. The passengers on this Caribbean “nakation” are mostly middle-class retirees, former teachers, lawyers and administrators, who cluster round the piano in the evening and ask for something by Elton John or Billy Joel. During the day they sign up for sessions on everything from pastry-making to wine tasting by way of ice carving. The yoga class, oddly, is pretty much the only place where clothes are compulsory, since the prospect of 100 naked people simultaneously doing downward dog in a small space is considered a breach not of decency but aesthetics.

The cruisers on the Big Nude Boat represent old school naturism, the type that is dying out and, its critics maintain, killing the movement: in Britain, the number of “official” naturists has declined by a half in the past decade. These college professors with neatly trimmed beards and penises swinging “like fleshy metronomes” to the beat of “Crocodile Rock” are following what is called “non-sexual social nudism”, which holds that not wearing clothes is natural, pleasurable and entirely innocent of any desire to gawp, titter or touch. Indeed, according to various manifestos (naturists are very keen on writing manifestos), social nudism is aimed at desexualising the human body. If it’s all on show, the theory goes, you might as well get busy with that on-board class in “how to maximise the effectiveness of Windows 8” with nary a glance at the naked person next to you.

And, actually, Haskell Smith’s experience on the Big Nude Boat and in similar resorts on land suggests that the non-sexual social nudists have a point. After a while, seeing naked people en masse becomes “dullsville”, and the only frisson he experiences when shopping in a nudist convenience store concerns the protocol of squeezing past someone in the narrow aisles. The big challenge is not, as Haskell Smith had feared, pesky erections but finding a suitable “total” sunblock to protect his most delicate parts (he is Scottish by descent and freckly).

It is not until Haskell Smith gets naked in the Austrian Alps, though, that he starts to draw close to the founding philosophy of the modern naturist movement. Nudism first became a fad in Germany after the first world war, promoted by radicals such as Richard Ungewitter, for whom the desire to be at one with nature, shake off economic slavery and eliminate social distinctions was best achieved by shunning textiles. All of which sounds reasonable until you learn that Ungewitter threaded eugenics through his philosophy, something that only became apparent with the rise of nazism. Chillingly, Haskell Smith describes how a young Hitler was torn between what he saw as the socialist decadence of Weimar’s nudist clubs and the opportunities they offered to identify Jews.

Facebook Twitter Pinterest Shoppers in the Cap D’Agde supermarket … the only frisson Haskell Smith experiences when shopping in a nudist convenience store concerns the protocol of squeezing past someone in the narrow aisles Photograph: Louis Quail/Corbis

The subtitle of Haskell Smith’s engaging book has him as a “reluctant nudist”, which should mean that, if he’s going to feel comfortable anywhere, it would be among those who make a point of practising non-sexual social nudism to the letter. Yet, oddly, he reports finding himself increasingly freaked out by what he calls the “strained Kabuki stateliness” of a community where naked people try so hard not to be sexy. Everyone makes an exaggerated point of holding eye contact, never allowing their gaze to drop to a stranger’s groin. Anyone found making out behind the sand dunes with their partner is told to pack their bags (a quick process, presumably) and asked to leave.

There is, it transpires, something approaching a civil war raging between the non-sexual social types and those who take a more libertarian approach. The godfather of the latter camp is a Frenchman called Émile Armand, who in 1934 published his treatise “Revolutionary Nudism”. This exhorted people to get naked and jiggy with strangers as a way of advancing a raft of social and political freedoms.

How this process of emancipation was supposed to work is unclear, but Armand’s “let it all hang out” approach to the human body is best seen in certain parts of the Cap d’Agde resort in the south of France. By day, multi-generational family groups lie out on the beach and play – yes, they really do – naked volleyball. But, come midnight, the hedonists get their freak on by teaming up in groups of three and four and giving each other oral sex while a machine blows soapy bubbles at them. Men wear leather skirts while the women favour masks and fishnet underwear. Having previously found himself on the receiving end of suspicious looks as a man nakationing alone, Haskell Smith now finds that, if he is noticed at all at Cap d’Agde, it is with inquiring interest.

The naked truth about San Francisco's nudity ban | Gypsy Taub Read more

Not that he has any intention of joining in. Haskell Smith repeatedly makes the point that he has a wife at home in LA, one who is understanding enough to chortle at his daily reports of the sexcapades on next door’s balcony, but who insists on proper boundaries. His stance throughout this project, then, is that of a participatory anthropologist. He is prepared to “put skin in the game” by getting naked around strangers, but he isn’t going completely gonzo.

The approach works well. He manages to avoid seeming either like a voyeur or someone who has a suspiciously convenient epiphany about the absolute rightness of nudity two-thirds of the way through his book. In the set pieces that describe his encounters with various degrees of nudity, he weaves an account of its historical manifestations and explores those sore spots in which the desire to be naked butts up against the law.

For instance, there’s the controversial 2013 ban on public nudity in San Francisco, spearheaded by the unfortunately named district supervisor Scott Wiener. This was upheld by a district court judge who dismissed the claim that nudity was a form of speech by arguing that “nudity in and of itself is not inherently expressive”. Or the case of Jeanine Biocic, who in 1989 found herself up on a charge of public indecency when she took her bikini top off while strolling along a protected wildlife beach in Virginia. She appealed on the clever grounds of gender inequality – men are allowed to be bare-chested, so why not women? – but the court wasn’t buying it. Meanwhile, in Montana they don’t even bother with such niceties. Get caught skinny-dipping three times, and the judge can put you away for life.

• To order Naked at Lunch: The Adventures of a Reluctant Nudist for £7.19 (RRP £8.99) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.