Not only that, the Whanganui has great swimming stories.
It was Bill Guzwell, a New Plymouth postal worker, who set out to swim the more than 250 kilometres from Taumarunui to Whanganui in 1953, with only his togs and a packet of matches.
He made 30km on the first day, then rain began, and he disappeared, leaving the New Zealand public, who were following the story, in suspense.
On the fourth day he was rescued, very cold, by a riverboat. Homing pigeons were dispatched with news he'd been found.
Then there was swimming instructor and pool caretaker Bert Olds, who learned to swim in the Whanganui River. In the early 1900s he put on shows attended by thousands - with races, diving and synchronised swimming.
One of his feats was the Monte Cristo - he would be thrown into the water inside a nailed packing case, escape, and swim free.
Conservation has been Lees' career. In her business she travels New Zealand, running meetings and workshops.
She often asks the people whether anybody likes taking a dip in the river or sea. Sometimes everyone hand in the room will go up, and she reckons there's nowhere in New Zealand without outdoor swim spots.
The book is her diary of swimming every day for a year, plus the swimming stories she gathered on the way. She tried a lot of swim spots, and said one of her best short bathes was in Wilkie's Pools on Mount Taranaki, in midwinter.
"There was a rim of ice, and snow lying on the edge. It was the elixir of life, the lovely fresh feeling of being in cold water."
But the very best swim is always going to be your local one, she said.
"You can't beat those because they are local, and they're yours."
The quality of the water New Zealanders swim in is now measured and scrutinised, and some of Lees' work is involved with that. People are realising water quality is declining, and she said it will take money spent in towns and on farms to improve it.
"We all own it and we all have to do something about it, and it's really, really worth doing something about."
Adults may consider their swimming days are over, but she said children still need those watery experiences.
"There is nothing like being lost for hours on the edge of water and playing with it with your friend and being physical in a beautiful spot."