Otherworld (2003)

Whilst celebrating Lleu's 18th birthday, Lleu and two friends go for a boat ride, when they discover beneath the water the golden doorway to the Mabinogi (the Otherworld). They swim down and are transported back in time many centuries to the Mabinogi. There they encounter some problems with mediaeval life, and they have to deal with the conflict between fate personal choice and magic.