Here is the full list of all 21 The Spanish Privateer achievements.
Join the crew
Attain Flint's main route
Attain Flint's second bad ending
Displease the crew
Attain Lark's main route
Attain Lark's first bad ending
Silence the songbird
Attain Rico's main route
Force a tough decision
Survive a mutiny
Collect all achievements