IN THE summer of 1996, while swimming in the moat of his 16th-century farmhouse during a downpour, Roger Deakin had an idea: “The notion of a long swim through Britain began to form itself. I wanted to follow the rain on its meanderings about our land to rejoin the sea, to break out of the frustration of a lifetime doing lengths, of endlessly turning back on myself like a tiger pacing its cage.”

He had always been thrilled by being “in nature, part and parcel of it, in a far more complete and intense way than on dry land”. He loved the feeling of freedom that came with nakedness and weightlessness: “I can dive in with a long face and what feels like a terminal case of depression”, he said, “and come out a whistling idiot.”

What resulted was Waterlog, a 300-page record of his journey between 1997 and 1999 from his home to the Hebrides via streams, fens, pools, lidos, estuaries and canals, sometimes with friends but often on his own.

In the book he remembered his mother cradling his head as he kicked his legs in the water, and visits to the beach as a boy enlivened by his News Chronicle I-Spy guide.

He described what he saw on his travels with elegance and humour. While with an eel trapper he noticed that “Every now and again an eel spilled on the bottom of the boat and slithered in reverse, then forward, curling itself into a question mark as if to say: ‘What the hell is going on here?’ ”

An erudite guide, he told how Rupert Brooke was punished for snootiness by having his access to a mill-pool blocked — and then getting to it anyway through a hole in the hedge; and how George Bernard Shaw took a troupe of actors to the warm brine baths of Droitwich Spa each day before the evening show. He described his admiration of Henry Williamson, the author of Tarka the Otter, of “the beauty and ice-clear accuracy of his writing . . . which sprang from the natural world and his passionate concern to take care of it”. These were qualities of Deakin’s own writing.

Deakin was irritated by the increasing number of signposts, and places being “officially ‘interpreted’ ”. He was also angered by the bossiness that surrounded the use of rivers. When the Environment Agency gave warning of the risk of contracting Weil’s disease from the water, Deakin countered with recent research, which showed that there are only 2.5 cases of Weil’s disease associated with bathing and water sports per year in the UK.

Swimming was for him a subversive activity that gave him access to “that part of our world which, like darkness, mist, woods or high mountains, still retains most mystery”. Therefore it came as a great disappointment when, taking a dip in the River Itchen, he found resistance in the form of “two figures straight out of Dickens; a portly porter with a beard and an Alsatian, and a gangling figure on a bike, strawberry-pink with ire”, who told him he was trespassing. It happened, coincidentally, on the day that Chris Smith had said: “Let’s make a right to roam a reality!”

He later told one interviewer: “The authorities are divorcing us from our birthright . . . Bathers once thronged our rivers and seas but they are becoming a threatened species.”

Deakin swam on to Cambridge, where Jack Overhill had taught people a new stroke, front crawl, in the 1920s. He visited holy wells in the West Country and a hidden canyon on the moors near Garsdale — where he was thrown against the rocks by white waters that fell 200ft; and Belnahua, one of the Slate Islands off Argyll, where he was surrounded by dilapidated cottages and derelict machinery: “everything was ruined, except its wild beauty”.

Malvern prompted him to consider the effects of ice-cold water on the body; Dr Murray Epstein’s clinical trials, part of the Nasa research programme, showed that it lowers the blood pressure and cholesterol, and boosts the immune system.

When the book was published he received so many letters from readers expressing their passion for swimming that, in 2001, he submitted a memorandum on the cultural importance of swimming to the House of Commons.

Roger Stuart Deakin was born in 1943 in Watford, where his father was a railway clerk. He was educated at Haberdashers’ Aske’s School in Hampstead, and then read English at Peterhouse, Cambridge, where he was taught by Kingsley Amis. On graduation he started work as a copywriter for an advertising company but soon left to teach English at Diss High School. He bought a farmhouse, Walnut Tree Farm, near Mellis, which he set about restoring. He dredged the moat, planted a wood and made hay. Having an enthusiastic and sensitive appreciation of his surroundings he decided to try making films — not only about allotments and Newmarket stableboys but also about rock’n’roll in Essex and Hank Wangford, the country and western singer.

His new country existence fed into his writing. He wrote a number of programmes for Radio 4. One was an audio diary of a year spent tending his garden. In another he paddled down the Waveney in his canoe, Cigarette.

Deakin also put on concerts in East Anglia and was a trustee of the Gogmagogs, a music/theatre string ensemble. He was the founder and director of the arts and environment charity Common Ground, which organised an annual Apple Day, a celebration of the English apple. And in what spare time he had he made wooden sculptures that were greatly admired by friends.

After the success of Waterlog, Deakin decided on another trip — to see the ancient walnut forests of Kyrgyzstan. He also visited Belarus, Ukraine, Poland, Lithuania, the Czech Republic, and Kazakhstan, encountering along the way savage dogs, a wolf and a gun-wielding peasant who thought he was a bear. He finished his account of the journey, Wildwood: A Journey through Trees, before he became ill.

Deakin was a kind, modest man with many friends. His pupils continued to visit him years after he had taught them.

Roger Deakin’s marriage to Jenny Hind in 1973 was dissolved in 1982. He is survived by their son and by his partner, Alison Hastie.

Roger Deakin, writer and environmentalist, was born on February 11, 1943. He died from a brain tumour on August 19, 2006, aged 63.