Hey there.
I only wear a swim suit when I have to, that is, at non-clothing optional swim areas, mostly beaches, because I don't swim in pools very much these days. In order to be as naked as possible at these places I only wear speedo-type swim suits. They are almost as good as being naked, because they are as skimpy as possible.
I do own underwear, and I wear them when clothed. But that's only because I have a cartoon undies fetish ;-)
Cheers.
I always take off my underwear before swimming, whether at home, or at a nude beach or resort. When I'm at home I don't wear any underwear either. In my experience a lot of guys strip down to their underwear when at home and often live that way, but I don't stop there. I take off my underwear as well and live completely naked.
I never swim in underwear.
Though my preference is swimming nude, I do swim in underwear when I have to wear something. I have several pair of sports briefs that I use when in a public setting. They are made from a lycra fabric that stretches and moves with me, snug but not tight and restricting, and so much better than the long baggy things that are available for men these days. Not too different from the suits I wore in the 60's.
Have done both at times for years.
Gradually stopped wearing underwear all together and got rid it all after I retired.
Last summer was the first time I only went to the clothing optional beach.
Also I think it was the first time I swam more without a suit than with one in my backyard pool.
I need to wear a suit when we have company because relatives and friends are mostly non-nudists.