This holiday commemorates the baptism of Christ in the Jordan River, which explains the custom of boys diving into the waters on that winter day (sea, lake, river) competing to find the cruxifix tossed there by the local priest. Greeks, as everyone remembers, are to this day, a sea faring people.
All bodies of water are blessed by the priests, and in the church, fresh water thus blessed is brought home by the local people to their families to drink, to sprinkle on/in their houses, on their animals, crops, and wine barrels.
In seaside areas, many people bring farm implements or the family icon to wash them in the sea, recalling the ritual of the ancient Athenians who carried the statue of the goddess Athena to the coast at Faliron to purify it in the sea.
Going hand in hand with these rites of purification is the banishing to the underground of the 'kalikantzari', best translated as 'hobgoblins', both mischievous and evil, who run around teasing people during the twelve days of Christmas (25 Dec to 6 Jan). They are pictured as goat-like creatures, both dark and hairy, who get into houses through the chimneys (interestingly, just like Santa Claus in the west), with people building fires to keep them out and keeping the ashes later as charms against evil in both home and the fields.
The combination of hairy, goat-like bodies, and mischievousness point quite obviously to their symbolic connection with the pre Christian pagan earty deity Pan.