From the annual ritual of diving after the cross cast into the waters on the Solemnity of the Epiphany/Theophany.

From Epiphany at Wikipedia:

Following the Divine Liturgy, the clergy and people go in a Crucession (procession with the cross) to the nearest body of water, be it a beach, harbor, quay, river, lake, swimming pool, water depot, etc. (ideally, it should be a body of “living water”). At the end of the ceremony the priest will bless the waters. In the Greek practice, he does this by casting a cross into the water. If swimming is feasible on the spot, any number of volunteers may brave the cold winter waters and try to recover the cross. The person who gets the cross first swims back and returns it to the priest, who then delivers a special blessing to the swimmer and their household.

This photo is from the celebration of Solemnity of the Epiphany by Byzantine Catholics in the Ukraine. You can well imagine what the water temperature was like.