Well an air pocket works as long as there is not water moving around but if you are driving through it the pocket of air would not be there. Think of it like holding a metal brownie pan upside down under water. If you hold it still the air will keep the water out but if you slosh it around the air will be pushed out by the force of the water. The plugs however are the least of your worries. If you have put them in corectly the threads should seal them from being flooded by water. Its your alternator and distributor\coil packs, and on some trucks the ECU that would worry me. These have no protection and its not so much the water but the grime that comes with the water that will kill these parts. Or the vaccuum leaks that you might have would stall the engine and let water into the engine.

All in all, thats a great snorkle <img src="/forums/images/graemlins/kewl.gif" alt="" />