Personally, I thought Machi's first choice was pretty cool and fashionable without being overly racy, although I agree with Hibiki that she'd chicken out of actually going to the beach wearing it.
@KeiB If you are swimming naked, all you do is feeding the patriarchy, mainly the perverts. You also exposed your skin to stings and abrasion along with harmful bacteria. Wearing bikini or swimming naked would reduce the swimming speed too.