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The Princess Myrcella is one of Cersei's dromonds in service to House Baratheon of King's Landing. It is named after Princess Myrcella Baratheon by King Tommen Baratheon. He originally intended to name it the Moon Boy after the fool of the Red Keep, but Aurane Waters convinced Tommen to name the ship after his sister.
A Feast for Crows
The Princess Myrcella is among the ships being built as part of Queen Cersei Lannister's plan to increase the size of the royal fleet instead of relying on the traditional sea powers to protect the crown's interests. During her imprisonment by the Faith of the Seven, Cersei is visited by Qyburn, who tells the queen that Aurane Waters fled King's Landing with the new ships after her arrest.