Modesty in general depends on and reacts to the surrounding situation. One obviously has a different standard of dress for the dinning room, bedroom, and bathroom, yet modesty applies to all of the above¹. Modesty out of the home varies as well. For example, when "among the guys," yeshivah boys can swap their conservative code for jeans, high-tops and T-shirts—it’s tough to shoot hoops in slacks and black hats! But they wouldn’t walk the streets dressed like that for no money. Girls will do the equivalent—for their friends’ backyards or summer camps, they’ll exchange button-down shirts and pleated dresses for long-sleeved sport shirts and wide-bottom jean skirts… which they would not wear in public. In a sense we are all like that: we dress one way for a corporate meeting, yet quite another at a backyard barbecue. 

Similarly, girls go swimming and they will wear swimsuits. But while they’ll wear what they need to wear, it won’t be the racy, skimpy stuff prevalent today, most of which is designed to accentuate an appeal of a certain three-letter type. Because modesty in the street demands that you are properly clothed, and modesty at the pool means you are properly unclothed — changing in a modest fashion, wearing conservative swimwear, and only swimming with (and in the presence of) members of your own gender.