I went to the pool with ds1 the other day and sat in the spectator area because I wasn't able to swim.
There were the usual people in: kids, teens, older blokes doing lanes etc.
Then, three teenage girls got in - perhaps about 17 or so.
Two of them had bikinis on, but one had a black bra and matching knickers on. The male pool attendants thought it was Christmas and spent their time ogling her and trying not to giggle 
Why would anyone go swimming in their underwear? A swimming cossie or bikini aren't that expensive or difficult to get hold of.
Or is this what folk do these days? Haven't been to the pool for years.