THE PRESSURE OF WATER
To try it, you will need:
a deep bowl or sink filled with water
a plastic bag
First, stick your hand into the water. How does it feel? Wet, right? What else do you feel? Not much, probably. We are used to the sensation of having our hands in water, so you don't notice anything strange.
Next, put your hand into the plastic bag and then put it back into the water. Be sure that the water does not flow into the bag. What do you feel this time? Pressure! You can feel the water pushing the plastic bag against your hand. The pressure is pretty much the same as before (slightly more due to the size of the bag), but this time the water presses on the bag, which presses on your hand. This is a less familiar sensation, so you notice the pressure more.
Put both hands into the bag and spread them apart. This time, you feel more pressure and you can see the water pushing the sides of the bag inwards. As you move your hands apart, you may be surprised at how much pressure the water is exerting.
When you put your hand into the water, your hand pushes some of the water out of the way. You hand is now taking up space that was occupied by water, and the water is pushing back. When you put both hands into the bag and spread them apart, the water pushed back harder. If you measured it carefully, you would find that the water was pushing with a force equal to the weight of the amount of water that you push out of the way. When you spread your hands apart, the bag takes up more space, pushing more water out of the way. You then have more force pushing inwards and you feel more pressure.

HOW DOES A SUBMARINE DIVE:
Can you make a submarine dive?
Navy sailors do it everyday. Here is a experiment that demonstrates the principle that allows a submarine to dive. All you need is:
a tall narrow jar (like a olive jar)
a rubber sheet or cut up a balloon and use the end.
a medicine dropper (such as a eye drop dispenser)
A strong rubber band.
water.
Here´s what to do.
1. Fill the jar almost to the top with water.
2. Put the medicine dropper in and squeeze the buld just a bit. Allow only enough water into the dropper so that it is just floating on the top of the water.
3. Now, put the rubber sheet or balloon over the top of the jar. Stretch it tightly over the top of the jar and secure it using the strong rubber band.
4. Press down on the rubber sheet with your hand. What happens? Now remove your hand. What happens? Why?.
Here´s how it works.
When you press on the rubber sheet, the air has less space in the jar and so it pushes down on the water more. This increases the water pressure and adds more water to the dropper, this added water causes the dropper to sink. When the pressure on the sheet is released, the air resumes its normal space and exerts less pressure on the water. The extra water is pushed out of the dropper and it rises to the top.
Whether a submarine is floating or sinking depends on the ships buoyancy. Buoyancy is controlled by the ballast tanks, which are found between the submarines inner shell. A submarine resting on the surface has positive buoyancy, which means it has less density than the water around it and floats. At this time, the ballast tanks are full of air. To submerge the submarine must have negative buoyancy. Vents on top of the ballast tanks are opened. Seawater comes in thought the flooded ports and air is forced out of the vents, and the submarine will begin to sink. The submarine ballast tanks are now full of seawater, so it becomes denser than the surrounding water and begins to sink. The exact depth can be controlled by adjusting the water to air ration in the ballast tanks.