The Mauritius Command by Patrick O'Brian

Can Captain Jack Aubrey and his crew defy the odds, and outmanoeuvre the French, to take two small but vital islands in the Indian Ocean? Life ashore on half pay, despite the joys of family life, is unlikely to satisfy a man of action such as Jack Aubrey. The sea calls to him. And so, when his friend, ship's surgeon and secret agent Stephen Maturin, arrives with secret orders, Aubrey soon finds himself in command of a frigate and setting sail for the Cape of Good Hope.

But, in Nelson's navy, there are as many enemies within as without.