This one is out of the way and is found during the firefight with heavily armed henchmen. As you approach the first turret, head to the far right wall. You will see a hole in the wall. Swim through it and to the left. Climb up to the next level. Turn around and jump across to the other platform. You will see the treasure sparkling. You will be an easy target while retrieving the treasure, so either do it after you kill all the henchmen, or on Very Easy mode.