When you start the game enter the above key to unlock the cheats:
X triangle x square up down square left circle 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 triangle to lay down then you hold are and x 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.