Why do stranded marine mammals die so quickly?
In the case of whales, I always thought that it was something to do with the fact that they rely upon buoyancy to support their weight and this seems to support that view:


  When whales, including small whales or dolphins become stranded on beaches
  they suffer from the pressure of their own weight on their organs,in the
  water they are weightless.  They also suffer from overheating as they have
  blubber that insulates them in the water and outside of the water causing
  them to overheat.  This is why we place wet towels and cold water on their
  fins and flukes when do they strand to help keep their body temperature
  down.  Unfortunately most stranded whales do not survive once they have
  beached themselves.