Actually I've been swimming nude since I was a kid. It was normal for young boys to swim naked in the 1950s, sorta like our nudity didn't "count" because we were so young, like infants are sometimes considered today. I used our backyard pool naked until I was about 7 or 8, when my mother got me a snug speedo-like suit that did embarrass me, because it looked so "girlie". At the Jersey shore I got to wear looser trunks like my Dad, though.
Then also at 8 I went to boys summer camp for a few years, where we skinny-dipped, alongside our naked counselors. Now THAT definitely provided me an early education about the adult male anatomy! Some even got boners, first I'd ever seen.
I also went to the Y pool a few times in the late 1950s, which were mandatory suitless. But only during youth hours, never with the equally naked adult men, and our lifeguards had suits, unlike the adult periods.
At 14 in prep school we had a homeroom trip to another school that had a pool (we didn't have our own swimming program), and that was also nude. But by then I was having major problems with unwanted stiffies around the other guys, and I avoided male swimming pools for a few years.
But in 1968 I was required to swim nude at a college pool, when I was scheduled for some hydro therapy during men's hours following a motorcycle accident. And a couple of years later I tried a nude Army pool at Fort Knox when stationed there, better able to control my arousals by then.
But then no nude swimming until the 1990s, when I started using gay resort & bathhouse pools, where it doesn't matter if I get hard. And also began staying at a clothing-optional gay campground, and joined a gay naturist club. So that swimming nude feels perfectly natural to me, although knowing there are other nude guys next to me is still a special pleasure.