Most makers of jockstraps make a kind of jock that is smaller to fit underneath speedo-like racing briefs. So yes, use a jock if you are a swimmer. But if you mean swim in just a jock... well, because it doesn't cover your butt, you better have a swimming area that allow nudity or is very private. All I know is, I made the swim team and we got our speedos, then my coach said I have to always wear a jock under it. The team jokes about it now but it was really embarrassing. She said my junk was not really fitting in my briefs very good! So, good example of when to wear a jock swimming, I guess.

If the swim suit is too loose a swim jock is good to have. I do not have any experience with swimming in a jock strap by itself, but there are some new styles like this. It is sort of a variation of a thong swim suit. It sill be interesting to see if the jock strap style becomes popular? We will need some guys with more of a nudist background perhaps for this to work or swim pools that allow thongs can have jock straps? Anyone else with an answer?

Yes

I swim in a swim thong at my gym almost all of the time, now about 50 times over the last few years, so I tried a swim in my new swim jockstrap by itself as my new swim suit instead of my swim thong. My gym allows all styles of swim suits including thongs and jockstrap swimsuits for men and swim thongs and swim bikini for the women! My guess is that if your pool allows swim thongs than it allows swim jocks too. Swim jocks are usually made with bright colors like red, blue, green or purple and so on to set it apart from the traditional white jockstrap underwear style. They do a better job of holding your junk than a swim trunk.