Record how many washers it takes to sink it. If you have different-size washers, try to adjust the weight on the object so that the object is of neutral buoyancy. Look around your home for objects that float which you can sink with washers. Try to sink the balloon. Tie a paper-clip hook to the neck of the balloon and add washers.
Divers use balloons to lift heavy things from the bottom.
Invite a diver to come and talk to your class about diving. Ask the diver to bring a regulator and a tank so you can see what it's like to breathe through a regulator on dry land. In cold water, divers wear wet suits. These neoprene foam rubber suits trap a layer of water next to your skin. When you jump in, you feel the cold water seep into the suit, but your body warms up the water right away.