Just channel your inner Nao-san and go commando, Ran - chan

That's a naive idea!

Always carry your fishing rod, you never know when you'll need it.

I dunno why anime and manga is always so against using swimsuits as underwear, even going without instead of having to wear it under their clothes.
As long as it's dry, there's no problem. It shouldn't be any less comfortable than normal underwear, it'll just sit differently under your clothes.