How far underwater can humans swim without being crushed?

Water won't crush you at any depth if you are able to fill your body cavities (lungs, middle ear, sinuses) with compressed air at the same pressure as the water. If you don't have compressed air to breathe you will have a very difficult time going more than 500 feet down without suffocating, not from the pressure, but from the time it takes to travel that far while holding your breath. In any event, your body will not have been crushed because it is mostly incompressible water.

If you don't clear your ears (equalize pressure in the middle ear) you can rupture an eardrum in the deep end of a swimming pool.