Anyone come across women sunbathing in their street clothes, such as their underwear bras and underwear? I had that experience a few years ago. I went to a local beach after work to mellow out and sat down at a bench and noticed a woman on a blanket with friends in underwear. One woman turned over and laid up buns up in a thong. This was in a fairly busy area next to people throwing a fresbee around, two guys and a woman. They woman made a negative comment but the guys did not. I made soft of a "so what" comment to her and she shut up.

In Florida it is common to go commando under your shorts. Too hot for underwear.
However when going to yard-sales we often wore swimsuits, thongs or similar under our clothes, so we could strip off and try on a pair of jeans on the spot.

I don't own titie whites, and avoid swimsuits and thongs in white, or with white trim, or white binding on the edges, because they look more like underwear.

To the beach I will wear a suit that is the right style and color, no matter if it was intended as a swimsuit or underwear, so long it provides proper coverage.
If partly transparent or translucent then reserved for just around the home or on the boat.

I have seen girls wearing their underwear at the beach.  It is very apparent that it is underwear but it does not bother me.  It did not seem to bother others although one girl ran down and sort of stared at her.  You never know what people are thinking.  She could have just been processing it.  She just kept running by.  Although you could probably tell from up close that they are underwear, you cannot tell from afar.  

I don't wear 'tighty whites' on the beach.  However many of my 'thongs' or 'g-strings' I do wear on the beach, or in public, are marketed as underwear.  

The ones I don't wear are the ones with wide elastic waist bands boldly stamped with underwear brand names.  While colorful, these seem to derive from the 'tighty white' tradition, are often not designed to minimize coverage, and when they are the rears turn out to be jock straps instead of thongs.

From the other direction, many Chinese made underwear is styled like swimwear.  The colors and patterns are like men's swimwear and swim-thongs. The fabrics are spandex or nylon rather than cotton. From a distance you can't tell if it is a swim thong from a US manufacture like PrevalSport or Skinzwear,  or if it is a Chinese knockoff.  

About the only difference is the Chinese 'underwear' thongs are often lighter: They are not the heaver bullet proof spandex of the 1980's swimsuits, like the iconic Speedo brand thongs. However US manufactures are also turning to lighter fabrics, for example the "Thin-skin" fabric of the aforementioned Skinzwear brand. So there is no practical distinction between 'underwear thongs' and 'swim thongs', except maybe in the colors and decoration.

Some of the Chinese color choices, often in dark red or strong blues, often with accents like 'racing stripes', look more like traditional swimsuit spandex designs than some of the fabrics used by US makers. 

Thongs in light colors that go transparent when wet, are cotton, or decorated with lace and frills, are obviously underwear.  Such thongs are more likely to make other people uncomfortable than they might otherwise be, because they are so obliviously underwear, and you are flaunting it. There seems more social prejudice towards thongs that are obviously underwear than ones that look like swim thongs.

On the other hand most people will politely ignore a person who has come to a beach or lake without being prepared to swim, who then strips down to their underwear to take a dip. And with all things thong, women face less prejudice for appearing publicly in underwear than men do. Many current styles of women's swimsuits having lace inserts and other underwear crossover styling. As Tobias said above, for men, the key is picking the right colors, fabrics and fit. 

Thonged4Life

See this thread:  http://thongboard.aimoo.com/topic/Thong_deals__suggestions_and_recomendations_from_JM_Runs-575880-7356-1