A swim cap is not a luxury to a competitive swimmer who needs one to eliminate any iota of "drag" as she (or he) churns through the water. Once upon a time every girl and woman wore one, despite the fact that they didn't really keep your hair dry. They were pretty ugly, too—utilitarian white rubber helmets (with chin strap) purchased for $1.98 from the drug store.
Then came "swim chapeaus", imaginative toppers designed to show off your compressed skull with pizazz. They were all the rage in the 1950s, the cap of choice while enjoying the sand and sea or your local community pool.
These caps weren't cheap. It took a lot of baby sitting hours to save the $10. Mine was two shades of shaggy pink latex, similar to the one below. I was going for water sprite.
I loved Esther Williams' MGM musicals. Esther rarely wore a swim cap in her elaborately choreographed swimming routines. I wonder what they used on her head during those film sequences... bear grease??? She did sport a cute cap while frolicking with her children in her own (Esther Williams brand) pool.
You can find retro-style swim caps today, to be worn with a wink and a smile. They are colorful and fun. But the originals promised you would have the allure of a mermaid, or appear as a sea creature come to life. Keeping hair dry was completely beside the point.
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