Here is the full list of all 17 Buccaneers! achievements.
Fully upgrade a ship.
Have a full fleet of four ships (excluding temporary ships).
Complete Act1.
Have a fleet of two or more ships (excluding temporary ships).
Choose which faction to join.
Capture any town by defeating its fort.
Capture a ship.
Rename a ship.
Acquire a Three-Decker.
Steal a ship from an enemy harbour.
Reach player level 20.
Complete Act 2.
Complete Act 3.
Conquer the Caribbean for Britain.
Conquer the Caribbean for France.
Conquer the Caribbean for Spain.
Conquer the Caribbean for the Buccaneers' League.