Skinny dipping

In the first book, Mr Finchley discovers his England, there is an episode involving Mr. Finchley and a tramp at Blagdon Lake.

He looked about for a spot to undress and, a few minutes later, stepped from the rear of a blackthorn as naked as the day of his birth. To any observer the pallid white of his flesh might have proved a discordant element in the harmonies of blue and gold and brown. Mr. Finchley was not worrying about observers. His little bay was practically hidden from all except the hills opposite, and they were too far away to give him concern.
. . . he heard footsteps on the beach and a man came round the side of the blackthorn. He stopped in front of Mr. Finchley and eyed him with a truculent expression. He was a fattish, middle-aged man dressed in a long ragged raincoat from under which a khaki tunic showed. His face was tanned with sun and dirt and a straggling moustache joined forces half-heartedly with a three day growth of beard.
“You bin baving?” the man asked sharply.
Mr. Finchley smiled genially.
“Yes. It’s glorious in. Why”—the man’s appearance prompted the thrust— “don’t you try it?”
“Say! None of yet lip, cocky! It’s me duty to tell you that you bin baving in private water. I could ’ave you summoned.”

In the second book, Polycarp's Progress, Polycarp rescues the millionaire businessman Joseph K Winterton who has fallen asleep while sunbathing in the nude and has his clothes stolen by a tramp.

In the third book, Fly Away Paul, Paul Morison finds himself having to shelter behind a partition to hide his nakedness from the approaching Margaret Sinclair.

A wave of panic flooded through Paul. He thrust up an arm and shouted “Stop! Don’t come any closer!”
Margaret stopped short and eyed him with wonder. “Whatever is the matter with you?” she asked curiously.
“Please …” He waved a worried hand over the top of the partition. “Please stay where you are. It’s not safe … It’s …”
Margaret frowned and half smiled. “Anyone,” she said, “would think you had the plague.”
“It’s worse than that,” confessed Paul.
“Worse than the plague? My dear Peter—what are you talking about?”
Paul saw there was no gracious way out of the confusion. He swallowed nervously and said: “I’ve got nothing on.”
Margaret looked at him, and a little fleur-de-lis of wrinkles decorated her forehead.
“You’ve what?”
“I’ve got nothing on!” repeated Paul. “I’m naked!”
“ Naked?”
“Yes, naked. I haven’t any clothes. I was caught in the storm.”
“Haven’t you even got a pair of trousers on?”
“Nothing. I tell you I was caught in the storm.”
“But you don’t usually go about in storms with nothing on, do you?” asked Margaret, and there was a wicked light dancing in her eyes.

In Two Men Fought Hilda Selyac strips off and swims naked in the sea in a deliberate but futile attempt to seduce Stephen Cornelly.

In Every Creature of God is Good the farm girl Lena comes across Godwin swimming in a pond.

When the man came into the shallow water he changed to breast stroke and a few seconds later he saw her. He stopped swimming and let his hands sink to touch the bottom and support him, his head and shoulders rising from the water. For a moment or two he stared at her as though he expected her to make some move.
Lena looked back at him, her eyes on his shoulders.
“I’m afraid,” the man said after a while, shaking his hair to free it of the dripping water, “that I shall have to ask you to go away while I come out of the water. I wasn’t expecting company and I haven’t a costume on.”
“I’ll look t’other way,” she answered, and she turned round, her eyes finding the grey roof of the farm across the meadows

In Everyman's England there is a chapter on Oxford and the nude bathing area Parson's Pleasure.

In Fountain Inn Ben and Helen Brown have to strip off when they swim across the lake to the island in search of Miss Logan. Ben says:

"I wish I’d got a costume with me. But it’s dark enough for me to undress without being immodest. You’d better stay here and watch my clothes.”
“I’ll do nothing of the sort,” said Helen vigorously. “I’m coming with you.”
“There’s no need for both of us to go in,” protested Ben.
Helen ignored his plea and began to slip off her coat. “You needn’t think, Benjamin Brown,” she whispered intensly, “that you’re going to have all the fun of swimming out to that island alone while I stand here doing nothing.”
“I was only being chivalrous.” Ben began to strip.
“I wish I could believe it. You didn’t want me around. I know you.”
Ben said no more. In a few moments they were both ready. Ben in his ultimate underwear and Helen, he guessed, trying to persuade herself that a silk shift would make an excellent bathing costume. Had it not been that they both thought they might find someone on the island—Miss Logan, perhaps—they would have discarded even these flimsy concessions to a convention which the night rendered unnecessary.

In Panthers' Moon The hero is in hospital after a train accident:

"Look, I want to get out of this place and come with you. I’m quite fit. Can you persuade Dr. Sergius to let me go? I don’t want to make a fuss, but he is being rather high-handed. They’ve even taken my clothes away.”
Anatol shook his head. “I can’t do that, Mr. Quain. After all, he is your doctor, and knows what is best for you.”
“I’ve no doubt he acts from very good motives. But I know I’m all right. What do they mean by taking my clothes?”
“It’s odd how our nakedness can be our own prison, isn’t it?” Anatol seemed to have forgotten Quain’s request for help. “From the moment of the Fall in the Garden we became the prisoners of our clothes.”

In The Great Affair Nelo remembers this incident from his childhood:

[My brother] was getting cross and if it went on I knew that he would begin throwing things. He always had. We were bathing together once, both stark naked, when some idiocy of mine had annoyed him and he had thrown a dustbin lid at me. The edge was jagged and cut me, leaving me for the rest of my life with a long, slightly puckered scar on my left side just below the ribs.

In The Doomsday Carrier Clarence Bedew, a retired civil servant, is enjoying a morning swim in a river when Charlie the escaped chimpanzee picks up his clothes, plays with them and starts throwing them in the river.

In the story "The White Spell" one of the three brothers will be obliged to marry Helen since she has been bathing naked in a mountain stream and they have seen her "as only a husband should".