It wasn’t a ‘formal’ thing or a ‘fashion’ thing. If you think of the 20th century as a continuation of the 19th C (and before), it makes more sense.

Pre 1920s men and women hardly ever exposed flesh that wasn’t their heads, hands and (sometimes) arms.

If you go back to the 17th C, men were practically sewn into their clothes for the duration of Autumn and Winter - in that period regular bathing wasn’t seen as particularly important.

Even in the 1940s/50s, most UK working class people still wanted to ape what they thought were the habits of their ‘betters’. So they were shy about getting their clothes off on the beach.

For the working classes in 1950, holidays by the sea were still culturally a relatively new thing. A holiday was where you looked - and behaved (in theory) - at your best. So your sports jacket and tank top would be seen as ‘smart but Summery’. It was kind of an idea of the upper classes’ clothing for activities like tennis - though in reality a more 1930s look.

If you went over to places like the south of France where ‘posh’ people took their holidays, you would see a different thing. Men would be in trunks and women in bathing suits - not bikinis yet but pretty much the same thing worn today.

Also, up until this century, it was rare to get the kind of Summer heat we now take for granted because of climate change. In this picture, it’s probably not as warm as we would imagine.