Quite possibly my favorite of the Aubrey-Maturin novels so far,
The Surgeon's Mate (the seventh in the series, if you're keeping track) is another winner for Patrick O'Brian. A Fog-shrouded chase through the Grand Banks, a secretive mission into the Baltic, a prison-break, and lots of the usual nautical patter and witty asides that make O'Brian's characters so likeable and his books so much fun to read.