Swimsuits Parade, Sydney, Australia

Female bathing costumes were derived from those worn at Bath and other spas. It would appear that until the 1670s nude female bathing in the spas was the norm and that after that time women bathed clothed. Celia Fiennes gave a detailed description of the standard ladies' bathing costume in 1687:
The Ladyes go into the bath with Garments made of a fine yellow canvas, which is stiff and made large with great sleeves like a parson’s gown; the water fills it up so that it is borne off that your shape is not seen, it does not cling close as other linning, which Lookes sadly in the poorer sort that go in their own linning. The Gentlemen have drawers and wastcoates of the same sort of canvas, this is the best linning, for the bath water will Change any other yellow.
The Bath Corporation official bathing dress code of 1737 prescribed, for women:
No Female person shall at any time go into a Bath or Baths within this City by day or by night without a decent Shift on their bodies.