Here is the full list of all 27 Aquaria achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Find all the recipes in the game.
Find all the Ancient Turtles.
Travel to all the areas of Aquaria.
Learn all the songs in Aquaria.
Rescue all the lost souls.
Defeat Mom and Dad.
Find all the health eggs.
Reach the Frozen Veil.
Discover two peculiar-looking fish...
Enter the belly of the Whale.
Defeat the King Jelly.
Beat the Arnassi race in less than a minute.
Defeat Octomun.
Defeat Mergog
Ride Ekkrit for a minute.
Defeat the Priests.
Fling a Monkey.
Kill the Coward.
Dive into the water from a certain high location...
Devour a Parrot.
???
Reach the Sprite Cave.
Defeat the Rock Crab.
Defeat the Mantis Shrimp.
Devour 6 creatues in a row.
Reach the Veil.
Reach Open Waters.