Looks to be some Russian Orthodox ritual. Some of the men are wearing bikinis, both swimwear and underwear.
https://www.youtube.com/watch?v=T4w1jKcOG8E
Interesting video
Re: Interesting video
A reasonable number of the men are wearing smaller swimwear, but since not all, I'm assuming you wear what you want to the Epiphany.
Other things I saw:
How clean everyone is (no tattoos);
Black, or at least darker colours, is a common colour for the swimwear, yet other colours are OK for towels and wraps;
The variations in the ritual: most go cross * 3, then plunge * 3, but a few go [cross then plunge] * 3. A couple did neither - forgot in the excitement of the moment or not Russian Orthodox perhaps. Another variation is that most keep their head above water, a few ducked under or washed the face.
Other things I saw:
How clean everyone is (no tattoos);
Black, or at least darker colours, is a common colour for the swimwear, yet other colours are OK for towels and wraps;
The variations in the ritual: most go cross * 3, then plunge * 3, but a few go [cross then plunge] * 3. A couple did neither - forgot in the excitement of the moment or not Russian Orthodox perhaps. Another variation is that most keep their head above water, a few ducked under or washed the face.