Why don't massive water-borne animals crush under their own weight when they're in water? If you take a giant whale out of the water and put it on land for long enough, it will crush itself under its own weight. Why doesn't the animal get crushed under its own weight when it's in water?
 A: One place this issue is discussed is here.  The key paragraph is 

Whales can withstand this pressure because their bodies are more
  flexible. Their ribs are bound by loose, bendable cartilage, which
  allows the rib cage to collapse to some degree under high pressure
  that would easily snap our bones. A whale's lungs can also collapse
  safely under pressure, which keeps them from rupturing. When the lungs
  collapse, the air inside them is compressed, maintaining a balance
  between inward and outward pressure. These adaptations are
  particularly important to sperm whales, which dive to depths of 7,000
  feet (2,133.6 m) or more, hunting for the giant squid that live at
  these great depths.

which is pretty cool. But then this leads to another problem addressed here: 

Collapse of the lungs forces air away from the alveoli, where gas
  exchange between the lungs and blood occurs ... the lungs of the deep
  diver cannot serve as a source of oxygen during the dive. Instead
  deep-diving whales and seals rely on large oxygen stores in their
  blood and muscle. Several adaptations enable this.

At the same time, as the article points out, the lack of gas exchange is useful in the sense that it circumvents the problem of nitrogen getting absorbed into the blood that would then bubble out at lower depths. 
A: When it's in water the buoyant pressures are distributed more evenly over the whale's natural surface contour, resulting in less internal strain in the whale's body.  
On land, the pressures are all concentrated in a planar surface at the bottom.  The whale's body is not naturally planar, so significant strain develops as the body attempts to conform to the planar surface in order to distribute the forces resisting gravity.  
A: The upthrust of the water is balanced by the force of gravity and also as previously mentioned a bendable cartilage in the ribs  helps sustain the high pressure 
