Underwear as swimwear
I very rarely wear underwear, preferring to be commando (Less clothing to remove when getting naked). However, I have from time to time worn underwear as swimwear, in the pool or the beach, or just "hanging out" on the porch. I do not have any underwear that is nearly as revealing as actual swimwear, until it gets wet. That can be fun...
When I first wanted some brief swimwear for a holiday I bought a yellow nylon g-string to wear as swimwear. It was great for swimming in and feels much better than any swimsuit I have owned.
More recently I bought some Kiniki transparent g-strings and have worn the black pair as beachwear on clothed beaches and the white pair for sunbathing and the hot tub on a seaside hotel's sun terrace.
I also have a black fishnet g-string which I have worn with a shirt to lunch at a beach cafe.