Black Betha is the ship captained by Ser Davos Seaworth. His son, Matthos is one of the crew.
Ser Davos sailed the Black Betha north from Gulltown all the way to White Harbor to distribute Stannis Baratheon's claim against the legitimacy of Cersei Lannister's children with Robert I.
Black Betha was part of Stannis's fleet that sailed into Blackwater Bay during the Battle of the Blackwater. She and the Lady Marya rammed the Lady's Shame, sinking her.