The epiphany is certainly a long established celebration in differing forms of christianity. Some lean more towards the magi while the orthodox seems more concerned with the baptism and "realisation" itself. Where and when the cross diving and releasing a dove started I've no idea, although perhaps it is the retrieval or rebirth of the cross that is significant? I read online that some places have the cross tied to a long piece of string in case the divers are unsuccessful, although I suppose this should only happen in very deep harbours.