I think you are misunderstanding a couple different things.My error is that I thought 1 bar is 0 meters and not 10 meters. I understand now, you are correct, the snorkel idea would not work.
At sea level, when you are at 0 feet of depth, you are under 1 bar of pressure. Every 10 meters of sea water you descend adds another bar. If you are at 5 meters, you are now at 1.5 bar--one for the atmosphere and 0.5 for the 5 meters of water.
If you dive at 16,000 feet of altitude, while you are at the surface, you are at 0.5 bar. Every 10 meters of sea water (I know, I know, it will be fresh) adds 1 bar, because water weighs the same at any altitude. So at 5 meters you will be at 1 bar--0.5 for the atmosphere and 0.5 for the 5 meters of water.