I mean other than flipflops that is. Welcome to Reddit, the front page of the internet. Showering naked is common etiquette in Iceland community spas. Popular with the oldies. Everything is very clean.
You only shower naked if there are individual shower stalls.
Taking a shower naked in front of the guy is right thing to do
However, I saw people doing so with swimsuits on. You're a wimp if you're not comfortable getting naked with a bunch of old dudes? Once you enter early adulthood though, wearing a bathing suit while showering in public is not only strange, but screams insecurity. I'm unconfortable being naked around anybody but my girlfriend, but that's due to a skin condition. Lots of older dudes would do a light workout and shower after work.