So if you were going to the pool, the beach, or something and by the time you got there to change (If you hadn't changed already that is.) and than you realized that you hadn't brought your swimwear, would you just change to your underwear and go swimming in your underwear? I would because swimwear and underwear are the same thing, literally the ONLY difference is that swimwear is aquatic and underwear isn't, but you can still go swimming in underwear even though it's not made for swimming. I don't want answers from you if you're a prude that's immaturely freaking out and like "Hell no! Why would anyone do that? Who does that? That's disgusting!".
It would depend on a combination of where I was and what underwear I was wearing. If I was at a family pool and was wearing a lace/see-through thong and bra set, maybe not (because obviously, this would be different than a bathing suit because it's see-through/shows a bit more skin. However if I was just at a friends place, than yes! However, when I swim at most of my friends houses we just skinny dip anyways as it's usually night time and no ones around!