I could have worn speedos, if all the guys did. But they didn't, so I didn't.
I agree with a number of the responses, that it's speedos, there's no hiding it. When I was on the swim team in high school. I always went for my towel or hopped into a pair of sweats. Not that I had an erection when I came out of the pool, but I was bashful. But when in sweats or in a towel, it's very easy to hide a boner.
I wouldn't have been bashful. I wouldn't care if people could see my penis bulge. And I'm proud to be circumcised, if they can tell? But I don't think that speedos were that popular, and I never had any, and I don't go swimming anymore, don't have time, and I haven't seen where my swimming trunks are now, for years.
So my "assets" will just have to remain a "mystery." Besides, some things really ought to be left a "mystery" anyway. If one is well-endowed, not many people necessarily need to know.
When I did used to go swimming, I think I may have stopped bringing a towel. What for? The sun dries one off in minutes anyway. And so if I got a hard on, oh well. Nothing around to hide it. Just have to try to tell it to relax and go back down, as I catch a few glipses of women's bikinis breast cleavage, to make not thinking of it, a challenge. I recall getting a bit stiff at times, but not so much as to very conspicuously show in swimming trunks. Besides, it's harder to see anything, when one is in the water.