Diving with your breath held simply squeezes your lungs and expands them again as you come up. No problem.

However, taking a breath of pressurized air deep underwater, and holding it coming up, is a recipe for severe damage (so is holding your breath during a decompression event in space). Under water, you let the air out slowly. In space, oddly enough, screaming is not a bad idea. Even if nobody hears you.

The invention of the Aqua-Lung used a regulator to adjust air pressure as divers went up and down, a significant advantage over old "pumped air" methods.

I don't know how much pressure change causes the bends. Obviously, it's more than what you get in an airplane.

Nick's idea of a bag isn't a bad one. A suit that would just be loose around you under normal circumstances, but inflate into a rigid "sitting" position under decompression. Having the inflated suit big enough to pull your arms in might be useful. Without worries about flexibility, you might even be able to get away with full pressure normal air.

And hey, there's always helium instead of nitrogen!