Dear enquiring minds,
     Actually the physics involved are pretty straight forward (ho hum
twas not my most exciting topic in college but).  As for the cold the
fat distribution, ie blubber on whales and such, presence of a certain
type of insulating fur, ie hollow haired or coated with oils to provide
a water resistant surface away from the surface of the skin itself,
actual core temperature required to survive and distribution of the
capillaries in the skin, effecting the radiative properties of the
skin's surface, contribute to the insulation and cold resistance of the
water mammels.  In humans we tend to distribute fat beneath the skin,
which can insulate the core organs but not the extremities, which has
too many capillaries exposed directly to the cooling effects of the
water without the benefit of a fur covering.
     The depth is not a problem for certain marine mammels due to a
different ear structure, ie equalization is not a problem, and
differences in pulmonary structure, ie the lungs, sinuses and such are
configured differently.  In humans depth reduces lung size to the point
where the volume occupied in the two hundred foot range leaves the lungs
and is compressed into the sinus cavities.  Once in this range the
gasses actually cannot exchange as they are not in contact with the lung
tissue itself.  Additionally below a certain level equalization is not
possible due to insufficient gas volume in the sinus, this may or may
not be overcome with certain techniques.
     As for length of submersion that is a function of metabolic rate,
ie how rapidly we consume oxygen, and lung volume.  In deep diving
marine mammals the respiration rate is lower than humans and
proportional lung volume and design is greater.
     Finally, speed and manuverability are partially aerodynamic, ie
friction is BAD, and mechanic, ie if you look at the way that aquatic
animals propel themselves and the way that these movements produce
forward motion you can see a big difference in the way that humans
produce motion in the water.  In a word these aquatic mammal guys are
just more efficient.
  Oh well keep wet and well.
Dennis
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