Or Else My Lady Keeps the Key

It's 1672 in Port Royal, Jamaica. John James, London bricklayer's apprentice turned pirate, is returning from the sack of Panama with his share of the loot (a lousy 200 pieces of eight) and a resolve to go back to bricklaying, since piracy pays so badly. First, though, he has a duty: he must deliver a letter to a lady.

The letter is from his dead comrade, Sir Thomas Blackstone, who was a court intriguer on a mission for Prince Rupert of the Rhine. The letter's recipient is Clarissa Waverly, Blackstone's mistress and accomplice. Before he went off to Panama, Blackstone hid four thousand pounds of the prince's money, unwilling to trust his lady friend not to make off with it in his absence. Dying of battle wounds, he wrote to let her know where he'd concealed the money.

Synopsis from Open Library.