Much of this detail on nude swimming came from Wikipedia:
In ancient and not so ancient times skinny dipping was the only method of swimming. Swimming suits had not been invented.
Benjamin Franklin, an avid swimmer, possessed a copy of the Art of Swimming by Melchisédech Thévenot, which featured illustrations of nude swimmers. Among other notable Americans, Presidents John Quincy Adams and Theodore Roosevelt are perhaps the best-known skinny-dippers. Quotations from the diary of Rev. Robert Francis Kilvert, an English skinny-dipper, in Cec Cinder’s The Nudist Idea, show the transition in the England of the 1870s from an acceptance of nude bathing to the mandatory use of bathing suits.
Skinny dipping was once very common in the U.S., especially for young boys and girls swimming in a secluded pond, swimming hole, or section of a river. Swimsuits were originally uncommon in these settings, as they were made out of materials such as wool that required extra care to deal with and were of limited practical benefit.
Although modern swimwear is more practical, skinny dipping remains a fairly common activity in rural areas, where an unwanted audience of outsiders is rather unlikely; yet it may be forbidden even there by law. Today, many swimmers in the U.S. limit their skinny dipping to private locations due to concerns about being nude in public.
Before the YMCA began to admit females in the early 1960s, swimming trunks were not even allowed in the pools, and high school swimming classes for boys sometimes had similar policies, citing the impracticality of providing and maintaining sanitary swimming gear and clogging swimming pools’ filtration systems with lint fibers from the swimsuits. These practices were common because of the perception that there was nothing wrong or sexual about seeing members of the same gender in the nude, especially in these indoor contexts among equals in ‘birthday suit uniform’.