Bathing in a gown and bonnet? Yup, they did that at the Roman baths in Bath!
If you had visited Bath during the time of Jane Austen, you would have enjoyed the privilege of using the public baths. Fully dressed.
In the book, Aristocrats, Stella Tillyard provides insight into what it was like to partake in the thermal water therapy using letters from the time period.
In the eighteenth century pride of place went to the Pump Room, where warm mineral water was sold by the glass, and the King’s Bath. This giant communal cistern was right under the windows of the Pump Room, open to the gaze of all. Patients sat in the bath with hot water right up to their necks. Men were enveloped in brown linen suits. Women wore petticoats and jackets of the same material. They sat side by side in a hot, faintly sulphurous mist.
Limp cotton handkerchiefs caught the sweat which dribbled down the bathers’ faces; afterwards they were tucked away in the brims of patients’ hats. Lightweight bowls of copper floated perilously on the water. Inside them vials of oil and sweet smelling pomanders bobbed up and down. On a cold morning the bathers in their caps and hats looked to the curious onlookers pressed against the glass above them like perspiring mushrooms rising into the thick gaseous air.
Thomas Rowlandson drew his humorous take on the bathing, although one assumes that he exaggerates the style of the attire.
I would not want to take my best feather-adorned bonnet into the sulphurous mist. I suppose there could have been such wasteful bathers, but I hope he was exaggerating for the purpose of comedy.
However, I can confirm that they wore headgear into the steaming water based on this account from Robert Warner, published in A New Guide Through Bath and its Environs, in 1811.
It is fit for the patient when he goeth into the bath to defend those parts which are apt to be offended by the bath, as to have his head well covered from the air and wind and from the vapours arising from the bath, also his kidneys if they be subject to the stone, anointed with some cooling unguents as rosatum comitiffs infrigidans Galeni Santo linum, etc. Also, to begin gently with the bath till his body be inured to it, and to be quiet from swimming or much motion which may offend the head by sending up vapours thither at his coming forth, to have his body well dryed and to rest in his bed an hour and sweat, etc.
Fortunately, today you can bathe in clean thermal baths in your swimsuit and leave your bonnet or tricorn hat in one of the provided lockers.
Annabel and the duke lived close to Bath, but I have a hard time imagining that either of them would have been interested in immodest public bathing while onlookers gawked at their sodden clothing, which revealed too much.
Annabel partook in bathing, but it was in private. I did a great deal of research to ensure the details were accurate so you can read about one of many private bathing rituals in The Duke Wins a Bride, when a sheltered daughter of a Somerset baron implores the duke to intervene in her doomed betrothal and winds up married to the philanthropic nobleman instead.