Breathing Underwater

A hard shelled jacket that keeps pressure of your chest would work, if such a device existed. What you have in mind is not too different from a magic bubble that does not burst with pressure. To keep pressure of your chest you would need something that hermetically protects your chest area to avoid the pressure on your lungs so they can expand and draw air into them. The problem with your jacket is that it is not hermetic, so water would flow into it and apply force on your body.

At 3ft its not possible to draw air from the surface?

Oh yes. Try it and see - when at school we thought it would be witty to make a long schnorkel, with a piece of hose attached to a polystyrene ceiling tile, floating on the surface of the swimming pool. I learnt a lot about atmospheric pressure that day...

In 3 feet of water the water pressure exerted on your body is a mere 1.29 psi neglecting the ever-present atmospheric pressure, sounds small right, well, if your chest were only 1 square foot in area, then the force on your chest is 144 times that or 187 lbs. That is a lot of force to overcome to suck in air.

The "hard shelled jacket" would need to be "air tight" at the sealing mechanism around the chest and would need an air line of sufficient diameter runing to the surface of the water.

Even if you had a sealed shell that would allow your lungs to overcome the pressure, there is still another concern with long snorkels. The longer the snorkel, the greater the volume of air inside it. If you are relying on your lungs to move air in and out of the snorkel, you run the risk of running out of oxygen. Not enough fresh, oxygen-rich air reaches the lungs, you simply rebreathe the same air. This could be overcome by a respirator-style device that would allow you to expel the used air into the water.

I agree with the comments above. A "hard-shell jacket" could, in theory, work as you suggest. However, in reality, the practicality of such a device would be very low. Movement would be extremely constricted. Sealing at joints would be very problematic, not to mention that extra weights would be necesary to overcome the bouyancy of the required air pocket that allowed chest expanesion (unless the jacket itself was that heavy).

A much better solution is available for those who do not want to strap SCUBA tanks on their back. There are small individual boat-mounted, or even floating, air compressor/hose/regulator breathing systems available for "skin diving" in relatively shallow water. The compressed air would overcome the pressure of the water allowing chest expansion, as well as provide the proper amount of oxygen, just like SCUBA gear, but without the bulk. Of course you are limited to the length of the hose and the capability of the compressor, but in shallow water, with a float-mounted compressor (or even pressure tank) that would follow you around on the surface, your mobility would be very good.

Personally, I prefer snorkelling over an area, then holding my breath and doing a surface dive if I want to see something close up further down. With fins you can really swim down quickly.

I went snorkelling with my brother and his fiancee one time on a dive trip they took in Pennekamp Park, Key Largo, Florida. They were SCUBA diving at the time, examining some coral formations, and I surpised them in about 15 feet of water by diving down to them on the bottom. I waived "Hi!" to them and my brother made some signs as if to ask, "Can you breathe down here?" Acting surprised, I made a choking motion at my neck and a funny face, then waived "bye-bye" and ascended to the surface!

Thanks to everyone for the input. I have a few new possibilities to explore now. Look out Nemo, here I come.

Old style divers on compressed air had a one way vavle in their helmet in case the air hose was severed on the ships deck or at some point in the water column well above the diver. This would keep full pressure on the diver. If the vavle failed the diver would be forced into the helmet and up the air line. Needless to say this was fatal, and divers always made sure their check vavle was in good shape lest they die of "the squeeze" as it was called. This hard shell about the torso, if connected to surface ambient, would have pressure on the head and legs to force those components into the shell. At 300 feet this would be about 150 PSI and lethal in an instant. At 15 feet it would not be tolerable for long, but not instantly fatal.

They are called iron lungs. However, they severly resrict your mobility.