Although never fully revealed to be true, Dart was most likely born as Rebecca's brother Dan. Years ago, he and Wil left the village on a journey. The two were separated in Badon and Wil never found him. Dan wound up washed up on the shore, but none of the villagers would go near him as it was a religious holiday in which it was taboo to touch blood.
Finally, Captain Fargus and his crew picked him up. He became an amnesiac, and was given the name Dart, and soon became Fargus's most trusted mate.