Bathing at the sea-side (or lake-side, or river-side or whatever happens to be near) is not a new invention. Our mental image of men and women in skimpy clothes, revealing more than they hide is not something new for the post WWII-era. People has done it for hundreds of years - though they did wear more clothes earlier on, that is true.
Bringing the camera to take vacation-pictures is also an old phenomenon, as can be seen in this picture from the shores of the Potomac river in the US. The photo is dated to 1921.
The woman of the left shows the most common head-fashion when bathing in older times - you wore a cap over it. They were not like modern ones that are meant to keep the hair dry since they were made of fabric that got just as wet as the rest of the outfit. There were many good reasons for women back in the old days to wear these little caps, in spite of the wetness. The most obvious one is that long hair was still common and it was the best way to keep it out of the way. But it was also a time when most women wore something on their head when they are out, it was the proper way to dress like a lady in the broader sense of the term.
The woman on the right is more unusual. She is not wearing a cap (though we of course don't know if she did not do that when she was actually bathing) and her long hair is flowing freely around her body. It is the 1920's, the decade of the short, boyish hair on women, but we are just at the beginning - and it is best to remember that not everyone cut everything off. When not bathing her hair must have been pinned up because grown women did not usually have their hair loose, not even in the roaring 20's.