The endless quest for the perfect bikini

We live in an immodest age. We do not go down to the waves in stripey huts on wheels - rather a shame, I think, since Victorian 'bathing machines' have always sounded fun for a ride, the Stannah stairlifts of the seas - but we must count ourselves lucky we do not have to wear wool to the beach any more.

In the mid-1920s, Vogue declared 'the newest thing for the sea is a jersey bathing suit, as near to a maillot [French leotard] as the unwritten law will allow'. This makes said suit sound like quite a liberated, daring item - far from the cold, wet, scratchy truth. I had to wear one once as a child, on the Venice Lido, filming a period drama set in 1927. It was a bit like an old-fashioned strongman's suit, a scoop-necked vest with a skirt concealing a generously cut pair of shorts.

Dry, it felt coarse but cosy; fine, as long as you tried not to think about previous owners. But as you walked into the sea it became ominously heavier with every step. Swimming in it was like fighting with a waterlogged cat. It thrashed against you at every stroke, threatening to drag you down. No wonder in their day they were known as 'suicide suits'. The only consolation was equality: the boys had to wear them, too. Everyone's nipples were shocking back then. Nowadays we have a lot more choice. Over a lifetime, we pull on and peel off several different Lycra identities according to our time and purpose: the good (that modest navy one-piece), the bad (sinfully small bikinis), the ugly (the tankini - half swimsuit, half cat's cradle, wholly wrong).

The First Swimsuit is all about conforming. It must be well-nigh identical to your schoolmates' or grief and trauma will ensue. The smallest deviance from school regulations - purple go-faster stripes down the side, for instance - will mark you out forever as a weirdo. Thanks to my mother, I had a standard-issue shiny navy Speedo, which I teamed with that most glamorous accessory, a verruca sock. I felt safe and shipshape in this kit. I liked it. Until it went mouldy when I left it in my locker over half term. 'Nothing,' said my mother, 'that a boilwash can't fix.'

The next phase is the awkward age, when we do not know freak from chic and are vulnerable to the predations of Novelty Swimwear, two words that should never be found together.

(Above) Hermione as a child in her 1920s bathing suit

We might be seduced by tan-through swimsuit material, a breakthrough invention that has bounced on to the market roughly once a decade since the 1970s ('left me with seams,' said a friend sadly). Or we might find ourselves sporting something like the tie-dye bikini, an abomination I wore in Jamaica aged 13. Sensing all was not well with this ensemble, I stayed in the shade, declining to enter the resort's 'best assets and swimsuit' competition.

Then there's the Dream Bikini (in my case silver and stringy) that marks your coming of age. Your post-A level, Greek island holidaywear, this is your glamour kit, your feminine armour. In it you discover the head-turning power of a halterneck and how to switch your beach presence on and off. Ten years later, you still can't bear to throw it away even though it's perished too much to wear. In your mind it remains the height of style, although scrutiny reveals it to be slightly dodgy, silver space-age tat from Morgan, made magical by experience.

Spending Big on something as small as a bikini seems absurd - consider the cost per centimetre squared of a £225 Missoni triangle bikini - until you go on a flashy holiday with older, ritzier friends and watch the women come down to the pool every morning in a different swimsuit, each more fabulous than yesterday, each accessorised with huge earrings, elegant sarongs... Your reliance on one frayed halterneck suddenly feels gauche. You consider investing in a vintage one-off in polka-dot silk or ruched rayon or, well, anything but wool.

But Fashionable Swimwear is a risky business. The style may be strong but the flesh is weak. After seeing Keira Knightley mooching around so elegantly in that white chin-strapped swimming cap in Atonement, looking like a Prada model sculpted by Brancusi, I was inspired to look out one of my own caps, kept from childhood. White and talc-dusty, it was a lot like Keira's. I used to hate the way it snapped the ears and left a red line across the forehead for hours after, but perhaps it could now be a style find... I pulled it on and saw myself transformed. Into a Smurf.

The last in the Six Ages of Swimsuit is the Faithful Friend, the swimsuit that covers, supports and flatters unconditionally, right through the giving-up-smoking bulge/the post-baby period/general times of insecurity. Possibly it is boned, or push-up (mine is) or engineered just so, or has a little skirt to cover bikini-line rash. It may not be particularly elegant but you feel comfortable in it, and its worth is more than rubies.