Original bathing costumes from the mid-19th century are vanishingly rare, but I did manage to find a few!
The example above is said to date to the 1850's and there certainly is a resemblance to the fashion plate from 1858 I posted previously.
Here's a couple from the 1870's; I especially like the Greek key trim on the second.
In some locations, suits could be rented - the photo below was taken at Coney Island, supposedly in 1865. Look at the right side of the sign "Fancy Flannel Bathing Suits".
Peter Tilyou and family moved to Coney Island in 1865 to establish the Surf House, a hotel and restaurant which sold Bavarian Lager for five cents. Tilyou treated those, who rented a bathing suit for 25 cents, a free bowl of homemade clam chowder on the theory was that those that were hungry would spend more money for food at his restaurant. He built bath houses nearby for those who wished to change from their city clothes into fancy full length flannel bathing suits. "Signs on the beach warned "Bathers Without Full Suits Positively Prohibited by Law."
I love this tinplate image, but it really makes me wonder - did these ladies choose to make matching suits or are they rentals?
Here's a CDV image - I was surprised at how short a skirt the lady on the left is wearing. The lady on the right almost appears to be wearing an outfit that was remade from an old dress, much like can be done when making a wrapper. Her trousers might have been made by cleverly piecing fabric salvaged by both shortening the skirt and removing a removing a breadth of fabric.
The question of attire for the gentlemen attending our bathing party has been an issue:
The commonly held reenactor notion, that men always swam in the nude is NOT an option and my research is showing that it's not necessarily always correct - I have found both descriptions and images of bathing suits for men.
Here's a suit documented to 1860:
And you've got to love this image - isn't he cute!
Here's an 1870 advertisement (and compare the lady in the center to the original 1870 suit above)
These gentlemen apparently shopped at Lord & Taylor's!
Salt water bathing was a rather novel idea in the late 1860's and 1870's. Those that came to the beach were apprehensive of even sticking their toe into the sea for there were those that warned that sea bathing might "leach away the essential salts of the body." They turned to a noted physician named Dr. Durant for reassurance. He advised that "the bathing dress should be made of woolen fabric...as it retains the heat of the body and therefore prevents a too rapid evaporation. Maroon and blue are the proper colors as they resist the corrosive and bleaching effects of salt water. Some authorities recommended twilled flannel, in browns, blues and grays, and stressed the desirability of broad-rimmed hats to fend off the sun and wind, as well as shoes to guard against the nipping of crabs.