Hydrophobia

Simply complete the game from start to finish. This doesn't have to be done in one sitting either. Although the game is kinda fun for the first 30 minutes or so it quickly loses its charm so I'd suggest going for the medals and documents as you make your first run of the game. This does disrupt the flow somewhat (no pun intended) but aside from some nice water physics and graphics this game doesn't have much to offer. Oh, and there's a bit of a cliffhanger at the end so if your not a fan of those then don't bother with this game.

This should only take about 60-90 minutes to unlock.

Hi, here is a All Collectible/Objects Collected walkthrough guide that i made :D. just a rare gem.
Act 1

Act 2

Act 3

Challenge Room - No Weapon Used.