Alright, here’s the thing:
I am a poor college student, and would like to take advantage of the pool available to me. However, I have no bathing suit, and as my clothing budget is limited to Wal-Mart’s bargain rack, I am curious if I could use some nylon exercise shorts instead of swim trunks. I picked up two pairs for a few dollars, and am going to test them in the shower. I’m really not sure what I’m looking for here. Is something going to (metaphorically) bite me in the ass if I wear nylon exercise shorts as swim trunks?
No, you’ll be fine. But:
Swimming pools have chlorine or other chemicals in them. These are hard on most fabrics. Some swimming trunks attempt to be chlorine-resistant, but no exercise shorts are likely to be, so you could be shortening the life of your exercise shorts quite signficantly (and almost certainly fading the colour, but that may not bother you).
Exercise shorts are less likely to be porous, and you may find that they trap water (or, indeed, air) causing them to “balloon” up around you, which looks a bit dorky and is an impedment to serious swimming. But a lot of swimming trunks intended for beach wear rather than pool training suffer the same problem. The fuller the cut/longer the leg of your exercise shorts, the more likely it is that this will be a problem.
Also swim trunks are designed with containment in a swimming environment in mind. The exercise shorts may also cling. Test them in the bath as well as the shower. See if they’re going to float you into trouble.
That’s the first thing that came to my mind. If you’re hung like one of ETF’s horses, you might need to stick with the Speedos.
Just wear a pair of briefs of a similar color under them.