Boys and their underwear

What about made for girl panties used either under swimming trunks in case they slide down or as swimwear by themselves? Or, in male only saunas where nude is ok?

Well in Europe boys mostly wear swimwear which are thin and small like panties.

When they are wet they are see-thru so they are basically walking around naked.

Yes indeed, in Europe, Britain and South America the Speedo or swimming trunks are favoured over swim-shorts. These are not always popular with boys, and not just for the embarrassment you mention of putting a bare bottom and other bits on display when you get them wet! A very common sight on British beaches is of a blushing and fretful boy with a T-shirt hastily pulled on and clutching it down over the front of his trunks, because the sight of a girl in her swimsuit has made him a bit too excited (if you know what I mean by that)! Meanwhile I can certainly imagine panties being used by boys for either swimming or saunas, and something else very popular with British schoolboys (rather more than they tend to admit, in fact!) and possibly in other countries too are panties for gym (also called PE knickers) worn under their shorts for physical lessons like basketball. In their full-fitting nature and their mixture of sturdiness and stretchiness they actually give boys more of that much-needed support than regular male underwear does. Boys here often find that in panties they can at least compete with the girls in mixed PE, if not beat them (girls are seriously outperforming boys in that lesson right now). Every boy should consider keeping at least one pair of PE knickers in his top drawer, just as long as you can stand the blushes of going into a shop to buy some!

I agree, PE knickers are irresistible to sissy boys. When I was a teen, I enjoyed the clinging feel of speedos, but was always curious about how girls PE knickers would feel. I didn't have the courage to go into a sports shop and buy some. Now we have e-bay and anyone can buy them easily without blushes. The best UK make is Gymphlex, if you want to search on e-bay for PE knickers.

PE knickers are an interesting experience for boys. Some get very fidgety at first and find themselves pulling and plucking at their gussets all the time, because they cover just so much and sit right underneath your bottom in a way ordinary boys' underwear doesn't. Ironically, that very same reason is why girls like wearing them! They also get you hot in the summer and can make boys feel rather trapped, so if you see a slightly fretful boy fanning his face and squirming a bit in warm weather that's probably because he's got PE knickers on! The ultimate though would surely be for a boy to wear old-school frilly-bum white tennis knickers to play a match in. Here's to Teddy Tinling (who designed the first ever pair, worn by Gussy Moran) and here's to the sissy boy, so much nicer to look at than the tedious regular kind!