I started wearing VS thong underwear a couple of years ago in the pool. Suit bottoms often dig into my skin in a yucky way and I like the way the underwear sit on the skin smoothly. Plus it's comfy and I think it looks good!
And surprisingly a member of the no panties in the pool club! The Mrs here also has been known to wear panties in the pool. She got a bunch of good ones at a stripper store that were obviously as tiny as a MS or WW for practically nothing.