"Image to yourself a small, snug, wooden chamber, fixed upon a wheel-carriage, having a door at each end, and on each side a little window above, a bench below - The bather, ascending into this apartment by wooden steps, shuts himself in, and begins to undress, while the attendant yokes a horse to the end next the sea, and draws the carriage forwards, till the surface of the water is on a level with the floor of the dressing-room, then he moves and fixes the horse to the other end - The person within being stripped, opens the door to the sea-ward, where he finds the guide ready, and plunges headlong into the water - After having bathed, he re-ascends into the apartment, by the steps which had been shifted for that purpose, and puts on his clothes at his leisure, while the carriage is drawn back again upon the dry land; so that he has nothing further to do, but to open the door, and come down as he went up - Should he be so weak or ill as to require a servant to put off and on his clothes, there is room enough in the apartment for half a dozen people."
(The Expedition of Humphrey Clinker, p.213)
Bathing machines, described in such a vivid way above, were a common sight in 19th century Britain. At the time bathing in the sea was considered a very healthy thing to do, in fact often recommended by doctors as a treatment for various illnesses, such as gout. Bathing machines may have been constructed in order to assist the sick and elderly into the ocean, but this wasn't the main reason for their existence. As any respectful upper-middle-class or high-class lady wouldn't dream of bathing in plain sight of other people, lest she be seen in what today would have been considered a ridiculously modest bathing costume, these machines offered a measure of privacy. It does seem, however, that in some cases people did actually use them to bath in the nude. At least one very well known type of recluse had a fondness for bathing machines:
The fourth is its fondness for bathing-machines,
Which it constantly carries about,
And believes that they add to the beauty of scenes -
A sentiment open to doubt.
(The Hunting of the Snark , Fit the Second: The Bellman's Speech)
Bathing machines seem to have emerged some time during the 18th century. Some sources claim they were invented around 1750 by Benjamin Beale, a Quaker who lived at a resort town by the name of Margate (The annotated Alice, p. 25 note 7), but there's at least one engraving from 1736 that appears to show bathing machines. It's possible that while the machine predated Beale, he added either a huge umbrella giving privacy on the back of the device, or a type of folding tent that sealed away a small section of the sea for the bather. These machines, it appears, while most common in the UK, were also seen on beaches in France, Germany, the US and parts of the British empire.
As bathing in the ocean, as well as bathing machines, were considered almost medical practices, and as some of the higher class people in the UK couldn't find their own ass with both hands, bathers were often accompanied by "dippers". These people, usually of the same sex as the bather, had the responsibility of getting the bather in and out of the water, and sometimes of making sure the bather was fully submerged at least three times for "medical" reasons. One of these "dippers, or "bathers" called Old Smoaker, responsible for bathing the Prince Regent during the rule of George III, is quoted as having the following conversation with prince George:
'I shall bathe this morning, Smoaker.'
'No, no, Your Royal Highness, it's too dangerous.'
'But I will.'
'Come, come, this won't do … I'll be damned if you shall bathe. What do you think your royal father would think of me if you were drowned?'
'He would say, 'this is all owing to you, Smoaker. If you had taken proper care of him, poor George would still be alive.'
Beside the Seaside, p.25
At 1901 it became legal for Women and Men to bathe on the same seashore, and bathing machines became less popular at a rapid rate. At the beginning of WWI some bathing machines could certainly be found, but they seem to almost have vanished entirely by 1920. A nice black and white photo of bathing machines can be found at http://www.dover-kent.co.uk/leisure/sea_bathing.htm.
Carroll, Lewis. The Hunting of the Snark . quoted at http://ingeb.org/songs/thebellm.html
Gardner, Martin, The annotated Alice, The definitive edition. Penguin Books, 2001
Smollett, Tobias. The Expedition of Humphrey Clinker. Penguin Books, 1967, quoted at http://www.jasa.net.au/seaside/bathing.htm
Walvin, James. Beside the Seaside. Allen Lane, 1978, quoted at http://www.jasa.net.au/seaside/bathing.htm