The Mary Cahoun is the smart brig that rescues Mr Duck from the island if Trinidade where he has been shipwrecked. The captain calls himself Jonas Fielder, but his real name is Robert Charles Bowline. Peter Duck says that the captain and mate had come up from round the Horn in some other ship .... the old packet .... and the captain and the mate of that other ship had died something sudden, and its come to me since that this precious pair I was with had taken their papers and their names at the same time.
She is sailing back to London, but is wrecked on Ushant Rock. Peter Duck is the only survivor; he says that they come home with a westerly gale and a full skin of rum apiece, and they piled the “Mary Cahoun” on Ushant rocks. "They" are the captain and the (unnamed) mate, who have been drinking heavily (PD5).
Captain Flint says he wouldn’t mind betting that Jonas Fielder was out of this life and in Davy Jones’s locker before Mr First Mate Robert Charles Bowline made so free with his name, took command of his ship and sailed her across to wreck her on Ushant (PD35).