air compressors, monoxide poisoning....

Water has weight. When we are on the surface at sea level - the air above us pushes down on our bodies. At 'one atmosphere' of pressure. This is roughly 14.6 pounds of pressure on each square inch of our bodies.

Water has weight. It's much heavier than air. When we go deeper into water, it presses in on our bodies. It compresses our lungs. It is very very difficult to 'lift' the weight of the water plus the weight of the air above it, with our chest muscles. You simply cannot 'suck' in air from a tube much longer than a snorkel. (A foot or two long)

Compressed air is forced down the hose to help balance the pressure of the weight of the water. The "REGULATOR" - normally held in the mouth while scuba diving - but can be attached to a faceplate in surface fed rigs - actually senses the pressure of the water around it and 'regulates' the flow of the air so that it reaches your lungs at roughly the same PSI as surface levels. So breathing while underwater is almost exactly like breathing on the surface.

I suspect that in the novel, the kids were either using a 'mechanical' air compressor - like you would find at a garage to fill tires or drive tools. These compressors take in air WITHOUT filtering it. So mechanical exhaust can be sucked into the air feed. Alternately - an improperly lubricated compressor will cause monoxide build up from the wrong kind of oil being used to lubicate the parts. When I learned to SCUBA dive back in the seventies, there were a couple of accidents because people were filling their takes at an 'uncertified' dive shop - who was using a plain old air compressor.

A certified diver usually doesn't dive without a buddy. Of course you could have your kids using equipment they found or discovered or something. So that there is only one person diving. IF you use a 'hooka' or surface fed rig - then sooner or later the brother on the surface will see that the line is not moving.

http://en.wikipedia.org/wiki/Diving_equipment

http://www.airlinebyjsink.com/

http://www.diversdirect.com/scuba/scuba-specialty-items/hookah-diving/

And yes, getting fouled in some sort of underwater obstruction is an excellent way to die. It very nearly happened to me.

Sometime, while ABOVE water, try to breathe only through a garden hose. Because your lungs only hold so much air, your expelled carbon dioxide can't make it all the way to the end of the hose, so you inhale it back in; the volume inside the hose is larger than the volume of your lungs. You will run out of air quickly.

Carbon monoxide forms from incomplete combustion, such as when a gas furnace can't get enough air or (which sent my cousin to the hospital) a welding shop isn't ventilated well enough. I find it a little hard to believe that a gas-powered compressor operating on an open boat will be starved for oxygen. For your story, I think a simple drowning will suffice.