Al Alvarez's swimmer's diary, describing his all-year-round swimming in the outdoor ponds on Hampstead Heath, north London, is a marvellous book. Even the title Pondlife is spot-on: unlaboured, light and right. But it has no business to be as invigorating and absorbing  its success is against the odds. What, after all, are the chances of making a good read out of a repeated activity, in the same place, day in, day out  a record of dips (most of which take place between 2002 and 2009) by an arthritic old man determined to free himself of age in the city's waters? In one sense, the book could not be more repetitive. But I read it as if each swim were a new question  which in a sense it was  and as if I were almost brave enough to take the plunge alongside him. And I never tired of hearing his answers  the sense of each swim as a tonic, a way of being born again.

Alvarez  once the distinguished poetry editor of this newspaper as well as being a writer, poet, poker player and former rock climber  reminds us that the same place is never the same. Like VS Naipaul in The Enigma of Arrival describing repeated walks in Wiltshire, Alvarez is alive to the heath's nuances  the pond the only fixed point throughout the changing year. The casual grace of his writing is partly what makes the journal such a joy to read: "Spring is back again, but pale, tentative, washed out, as though after a binge." I especially enjoyed his wry, neighbourly attitude towards birds  members of his swimming club. He salutes a mannish heron, keeps his eye on hazardous swans, describes Canada geese "ignoring each other, like some ill-matched married couple". He is a sort of water bird himself, although his ankle  a weak point since childhood  is too duff to allow him to be much of a wader.

The diary entries work like a swimmer's rhythmic strokes, pushing ahead through time. In winter, the water can be brutally cold. Oddly enough, just after finishing the book, I was walking on Hampstead Heath  on a punishing January morning  and was able to watch, from a long way off, an old man hesitating at the water's edge. It was obvious that the hesitation was about nerving himself to dive in. I thought of Alvarez and it made me realise I would have liked to hear more about how he learned to psych himself up to swim on the more intimidating days.

My suspicion is that there is some sort of tacit macho rule among outdoor swimmers that makes fear of cold a non-subject. Yet there are times when Alvarez emerges from the water with his teeth and jawbone aching and fingers too numb to do up his trousers or shoelaces. Fortunately, there seems always to be someone on hand to help. And what he does comment on  the only foregone conclusion  is that he feels better after his swim than before: pain temporarily banished. Swimming is also "good for the soul and cheaper than psychoanalysis".

Swimming is about living in the present and against the tide of age. Inevitably, the book is a reluctant meditation on getting old, although Alvarez does everything he can to ignore it (he is 73 when the book starts, eightysomething by the end). Age is treated no better than it deserves  as a troublesome hanger-on. He chafes against old bufferhood. He could not be clearer about the humiliations involved in not being able to get about easily. He lets us know about them in seething asides. He makes no secret of his vanities  the need for the adrenaline rush involved in overcoming a physical challenge that he once described as "feeding the rat". He wears his faultiness, frustrations and insecurities on his sleeve  or around his ankle.

He describes his loves and hates  his distaste for the literary world ("peopled by monsters") and his unfaltering love on a sudden sighting of his wife: "Forty years on and my heart still jumped with pleasure." He quotes Bette Davis: "Getting old ain't for sissies." And Beckett, who "got it right" when he wrote: "You must go on, I can't go on, I'll go on." His own motto is: "When in doubt, grit your teeth and do it." At one point, battling foul weather, he compares himself to King Lear without the Fool. I would say he is more like King Lear and the Fool together  with all the rash, rueful wisdom that implies.

Alvarez's tenacity is awesome. No wonder that, when he fetches up in Hampstead's Royal Free hospital, he rails against his captivity and gets a boost when his fellow swimmers pitch up with a jam jar of pond water as a gift. He is hanging on to what he calls his "pond cred". He turns 80 and reports feeling that he has arrived at an almost "absurd" destination: "I feel as if I'd been set free to do whatever I want." Even post-stroke, that means swimming. The book's last words are from Pancho Villa: "Don't let me die like this. Tell them I said something clever." Is it really possible he does not know he has?

Al Alvarez has always been one of literature's hard men: a poet, yes, but also a rock climber, a poker player, a scrutineer of suicides. On the face of it, his most recent book and, by his own admission, probably his last, lets him conform to type. It's a journal he kept between 2002 and 2009 (the year he turned 80), in which he describes more or less daily dips in the ponds on Hampstead Heath. "For a little while," he tells us, "between taking your clothes off and putting them back on, you are a 'naked, unaccommodated man', feeling the weather on your skin. It's a mild form of rock climbing: it strips away the comforts and protections that Shakespeare called 'additions'." Hard, see. Especially when it's snowing, the ponds are coated with ice, and there's a decent chance of getting attacked by a swan.

As things turn out, Pondlife is a good deal more nuanced and vulnerable than this. Sure, there are moments of wilful machismo, when he takes a swipe at "wimps" who stay away because the water is freezing. But the bulk of the book has very little to do with this sort of thing, being much more focused on the frailties of old age, on how swimming releases him from the pain of a crocked ankle that has dogged him for many years, and on the fading of lifelong pleasures. (He was six months old when his parents moved to Hampstead and he has swum in the ponds since he was 11.) It is, in other words, a journal of leave-taking as well as "a swimmer's journal", and parts of it  especially in the second half  are sympathetic and touching.

It requires a bit of patience to reach these rewards  not so much due to the male posturing, but because Alvarez seems to have begun writing the journal much more casually than he ends it. The language is pretty ordinary (the water is almost always "fresh and sweet"). The literary allusions are predictable (Lear and Yeats and Beckett for old age, MacNeice for "the drunkenness of things being various"). The main ingredients of the entries stay more or less the same: the same attention to the weather and the temperature of the water; the same glances at swans, herons and coots; the same rolling over to watch planes overhead; the same pretty leaves falling into the water (or not).

Occasionally there are breaks in the routine  when Alvarez goes to his house in Italy with his wife Anne; when he gets excited about a poker game; when he buys a new car; when he writes a review for the NYRB. But all these interruptions are made to seem just that, interruptions, which means that before he dives back gratefully into the water again, he doesn't take much trouble to expatiate on terrestrial life. This sort of concentration has produced very good books in the past, not least Thoreau's Walden. But while the pond in that masterpiece becomes a means of exploring the wide world, the water in Pondlife provides no equivalent sense of expansion.

And yet. Although Alvarez grumbles about age and decay from the start, his journal also changes as it goes along. Swimming itself becomes an even more urgent necessity  as a way of keeping death at bay  and the ponds become not so much "England's last outpost", manned by a bunch of hardy and likeable eccentrics, as a way of dramatising his defiance. Here, in a dim-lit world of wheelchairs, persistent pain, a stroke and the death of friends, existence becomes more precious as it grows more precarious. The plainness of Alvarez's language works to his advantage; it lets us see the courage in restraint. The same goes for the last few pages  which do not so much bring things to a conclusion as peter out, and (given the reasons) add to the poignancy rather than seeming an artistic failure.

In the process, Alvarez laments that he no longer has the energy to finish a book that he's been commissioned to write about old age. But he has written it, of course, and this is it  not so much Pondlife as "Swan Song". By the time it stumbles into silence at last, even the macho elements of the earlier pages seem altered  and seem a part of the understandable animal rejection of everything they cannot alter or restrain.