In Chapter 5: London Underground, after finding the car in the underground, you will enter the secret library. After a few cutscenes, you will get a map and encounter a small group of enemies who are shooting at you. Leave the library, and go up the stairs to your right (where you originally came from before entering the room). Go around the railing until you reach the midpoint at the end of the room. Jump down to touch the crocodile and get the "Ride The Crocodile" trophy.