The Happy Return (Hornblower Saga: Chronological Order #6) by C.S. Forester

June 1808, somewhere west of Nicaragua, a site suitable for spectacular sea battles. The Admiralty has ordered Captain Horatio Hornblower, now in command of the thirty-six-gun HMS Lydia, to form an alliance against the Spanish colonial government with an insane Spanish landowner; to find a water route across the Central American isthmus; and "to take, sink, burn or destroy" the fifty-gun Spanish ship of the line Natividad. A daunting enough set of orders, even if the married captain were not distracted by the passenger he is obliged to take on in Panama: Lady Barbara Wellesley.