Just a few comments on water pressure. Dive down 6 feet or so in a swimming pool, then hold your nose and blow out gently into the blocked nose to equalize the pressure -- just do this every 6 feet or so (you'll do this a lot while snorkeling or SCUBA diving) and you can dive down fairly deep before your lungs start to hurt (they will shrink by about half by 33 feet, etc.). The gases inside you are getting compressed so that the pressure inside equals the pressure outside, and with compressed air to refill your lungs you can survive even up to more than 1000 feet... but of course, highly pressurized air isn't the same as sea-level air, and gases start dissolving into your blood (the nitrogen especially). Too much nitrogen will poison you, plus you might end up with dangerous bubbles in your blood (causing decompression sickness) if you are down for too long and come up too fast.

Tie a weight to an inflated balloon and sink it -- it won't break, because the walls are flexible. It'll just shrink as the air is compressed until it looks totally deflated as it goes down. If you kept refilling it with compressed air, you'd be able to keep it inflated; just make sure you vent it as you rise or it will burst then. This is the SCUBA diver approach... when you're resurfacing, you just keep breathing out!

The other approach (e.g. the Trieste) is to avoid raising internal pressure to match external, but instead to counter the external pressure with a structure that can bear the weight. It's possible to go much deeper this way, as you can breathe normal air... but the pressures the craft must bear are incredible.