Here is the full list of all 12 Hydrophobia achievements worth 200 gamerscore. It takes around 6-8 hours to unlock all of the achievements on Xbox 360.
Collected all documents and objects.
Collected all the medals.
Neutralized 10 Malthusians in underwater combat.
Swept 3 Malthusians off their feet with water.
Completed the Challenge Room.
Completed the game.
Collected 50% of the medals.
Killed 2 Malthusians within a timed chain attack.
Killed 5 Mathusians with fire or electricity.
Drowned a Malthusian.
Saved Chief Billingham.
Completed a hack.