When you start the game enter the above key to unlock the cheats:
X up down left right L1 and R1
Crawl under water and on stairs
Its not really a cheat but It will help you in the game and people can't shoot at you but can hurt you by melee or grenades. So you get a grenade out and hold to lay down then you hold are and and don't let go of the buttons and just crawl under the water or crawl down the stairs but never let go unless you want to stand up.