We do what we have to do! I will usually wear a bathing suit. If it's just me and a few friends I don't mind going naked. I have worn underwear before, but I usually would go naked before wearing my underwear. I want something dry to wear for when I get out
I'm not sure about swimming naked, but I've never heard of girls swimming while wearing underwear. Even though swim suit pieces look the same, they generally aren't as see-through and are usually not as fancy as some panties are.