Probably a rokushaku fundoshi.
I've worn stretchy cloth breechcloths in the water myself. They've worked pretty well as long as the belt is water-resistant and tight enough to stay secure.
I do likewise, but mine's are blue and green.
Great for a day at the lake or river or beach.
I wish I had a place I could stroll around in a loincloth and not have to worry about others looking at me.
If by “something like that” you mean “clothing optional”, then, maybe, Germany (and Europe in general) may be a tad more liberal than the US, from what I've heard. But even here these places are not universally accepted or even tolerated.
For wearing loincloths, our fellow writer Aaron here seems to be able to find suitable locations at least in his part of the US.