Go forward and you'll soon find another destructible wall. Blow it up, walk through and go through the door on your left. Watch the cutscene and then hop down to the ice to rescue the cop. walk very slowly so you don't break the ice. If you hear a beeping sound, it's because you're moving too quickly. Once he's free, head back down to find the other two. head towards the right side of the room and be introduced to Penguin's friend. If you turn detective mode on, you will see golden glowing hooks scattered around the room and something hanging from the ceiling. Use the batclaw to grab and pull this object. It will fall in the water creating a raft. Shoot it with the batclaw and pull it towards you. Hop on and then use the batclaw on any of the hooks in the room to pull the raft towards them. Shoot at hook behind the cop and float over to his platform. The second guard is a little bit trickier but not overly so. You actually have to rescue him from the raft so make sure you get close.