Float a cork on some water, then place a transparent plastic container or glass over the cork. The cork represents a person and the container represents a diving bell.

Push the diving bell down into the water. You will see that the water does not go into the diving bell. (As the bell descends, the air inside keeps the water out.)

How does the diving bell make it possible for people to explore the ocean floor? Why do we even want to explore the ocean floor?

Here's another one:

Cover the bottom of a wide-mouthed bottle with BB's or pebbles. Pour melted paraffin over them to hold them in place. Put a U-shaped glass tube through one hole of a two- hole stopper and a short straight glass tube through the other hole. Attach a length of rubber tubing to the straight tube, insert the stopper in the bottle, and float the bottle in an upright position in a large container of water.

Suck the air from the rubber tube. (Water will enter the bottle, and it will start to sink.) Now blow air into the tube. (The water will be driven from the bottle, and the bottle will rise.)

Use this model to explain how a submarine submerges and rises.

Could we send an underwater vehicle into the ocean trenches?
Why are there trenches in the ocean?