I could be wrong... but. it seems to be just an ice bath. If it were baptism, one would assume there would be a religious officiant.

It looks like to be in shape of a cross or a cross is above the pool. There is another similar video with the pool in the shape of a cross, yet most of the young women jumping in have thong bikinis on....

A quick bit of google research suggests this is not baptism but an orthodox celebration of Epiphany, where believers dunk themselves three times in ice cold waters as a symbol of being cleansed from their sins. There is no obvious religious officiant present as the water is blessed beforehand. Full article here: https://www.cbc.ca/news/world/epiphany-moscow-photos-1.3409838 Aside from the content of the article, here’s an additional personal perspective from me: The fact that some of the young women are wearing thongs and no-one seems to care goes to show how cultural our responses to thongs can be. Wearing thongs has nothing to do with religion and faith, though context can clearly make a difference: here it doesn't, but in some other cultures it would.

In the summer of 2022 I went to a beach where a Church group was having a social event and a couple of young women, one in a v-string were laying out. No one said anything of those and other women in thongs but plenty of looks from the married men.