Invert a small test-tube or pen cap inside a 1.5 L plastic coke bottle full of water and screw the lid on. Have just enough air trapped in the tube or cap so it only just floats. You might have to add a blob of Blu-Tack or Plasticine. Screw the cap on and squeeze the sides. Your "diver" sinks! Let the sides go again and the "diver" rises to the top again.

Try this:

If you used a test-tube you will have noticed that when you squeezed the side of the bottle, the air bubble inside got smaller and more water entered the test-tube due the pressure of your hand. It displaced less water, so lost buoyancy and sank.

Letting the sides of the bottle go again allowed the trapped air (which was compressed) to push the water in the tube out again, so the tube became more buoyant, so it rose!

A submarine has tanks or compartments that can be flooded with sea water so it loses buoyancy and sinks. Compressed air stored in tanks elsewhere on the sub can be used to push the water back out again, so the submarine can rise to the surface once more.

The Lava Lamp picture above uses a different principle - that of density. The lamp has a heater at the base which heats up blobs of oil. The oil blobs expand slightly when hot, becoming less dense than the surrounding liquid, so they float toward the top of the lamp. After a while they cool, contract and become more dense than the surrounding liquid, so sink.

Some people think this is a good model to show how Magma (melted rock deep in the Earth, beneath the hard crust we live on) reaches the surface of the world. When the Magma reaches gaps in the Earths crust, it pours out as Lava and we have a volcano.