In Russian: купание голышом (нагишом).

German (noun): das "Nacktbaden"
as two words (verb): "nackt baden")

Baignade a poil - French

I'm Italian.
In italian you can say "bagno nudi" (it means: swimming naked) to mean skinny dipping.
Often the skinny dipping is performed on midnight, so you can find some stuff searching "bagno di mezzanotte" too (it means: midnight bath / midnight swim)

The first picture is a bit of a "Where's Waldo"

voyeur

Somewhere in russia, someone knows what kind of event it is?

more from this kind of event

part 2

Milf - part 1
 

Found it on Photofile.ru

Personally i find it a great series cause its like a kind of dare. 2 older women, at an abandoned place, and one of them is deciding if she will do it or not.

Lucky for us she did

PART 1