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.
Find all the health eggs.
Enter the belly of the Whale.
Travel to all the areas of Aquaria.
Discover two peculiar-looking fish...
Beat the Arnassi race in less than a minute.
Defeat Octomun.
Devour a Parrot.
Fling a Monkey.
Defeat Mergog
Kill the Coward.
Learn all the songs in Aquaria.
Reach the Frozen Veil.
Defeat the King Jelly.
Rescue all the lost souls.
Defeat Mom and Dad.
Dive into the water from a certain high location...
???
Reach the Veil.
Ride Ekkrit for a minute.
Defeat the Rock Crab.
Defeat the Mantis Shrimp.
Defeat the Priests.
Devour 6 creatues in a row.
Reach the Sprite Cave.
Reach Open Waters.