As far as I understand women had to be modest, and not show too much, and now that I'm at the beach I wonder when did that perception change to the bikinis we see today (probably mid 20th century) and how did women go swimming back in the day (if they ever did at all) because big dresses are not that comfy in the water I reckon.
In the eighteenth century, there was really no proper swimming costume - going into the water for health and relaxation was only just starting to be a thing. Around the middle of the century, Europeans started to get interested in visiting the seashore to be near the sea, and visiting natural mineral springs to cure ailments had been a thing for even longer; increased interest in the "natural" and in showing robust good health led to the idea of immersing oneself in cold, salty water as beneficial to the health becoming more and more widely adopted, and women needed something to wear. As a result, they basically went with versions of their underwear, adapted into something a little more substantial. What women wore under their gowns and stays were shifts, knee-length linen garments that soaked up the wearer's sweat and such. Typically, the wealthier a woman was the finer and lighter a shift would be - but for sea bathing, they wanted something sturdier. Martha Washington's bathing shift still exists, and you can see that it's a blue and white checked linen, inexpensive and well suited for aprons and towelling. Rather than having a deep neckline that would sit around the edges of a gown's, it's higher up on the chest and tied with a tape. The hemline is much lower than a shift's, since the garment would need to cover the wearer's legs to the extent that a petticoat would, and the hem has lead weights in it to keep it down in the water. Bear in mind that sea-bathing meant literally bathing, not swimming - the point was not to move around exuberantly but to just sort of be in the water.
Shifts continued to be the basic bathing dress well into the nineteenth century. You can see in the 1829 satire "Mermaids of Brighton" that women were still wearing unfitted long gowns, but with longer sleeves and higher necklines - which makes sense, as by 1810 the standard of long sleeves/high necklines in the day and short sleeves/low necklines for the evening if you were wealthy (if you weren't, it was long sleeves/high necklines forever) was pretty much set. One went sea-bathing in the day, so one had to cover up.
By the middle of the nineteenth century, going in the water involved more activity, which led to swimming costumes that covered the legs but also allowed for movement. This meant trousers! Trousers worn under short, often thigh-length dresses (as we would term them; they often called them "blouses") that generally followed fashionable lines in cut and decoration, made out of warm wool twill or flannel in blue, red, black, or white, which were supposed to be resistant to bleaching in the ocean. Caps were also worn to keep the hair together, keep the head warm, and conform to the social norm of women wearing some kind of headgear during the day. Let me share some adorable fashion plates:
By 1870, women were wearing breeches or knickers rather than trousers, often with stockings covering the bottom part of their legs - but it was acceptable to go without stockings, and also to sport short sleeves, likely depending on your level of activity. More plates, they're so cute!