I recently returned from traveling on my boat in Central America
accompanied by my two ferrets.  Here are three 'cool' ideas that seemed
to work well.
 
It is helpful to have a thermometer in the ferret area since humans aren't
always the best judge of temperature, especially when we are perspiring
and in a breeze.
 
The best method, since you don't have to cage the ferrets, is to use the
re-freezable plastic 'bottles'.  Simply freeze them, put one or two in an
old sock, and place it where the ferret likes to sleep or curl up.  You
could also freeze a plain plastic bottle with water, but the commercial
ones stay cold longer.
 
Secondly, confine the ferreets to their cage.  Put a damp towel on top
of the cage and aim a fan at the towel, not the ferrets.  The evaporation
has a cooling effect.
 
Thirdly, if things are really desperate, set a cage on top of a pan of
ice.  Make sure there is a hammock available up above so the ferrets can
escape if they get too cold.
 
Putting water on the ferret only helps for a short time as they seem to
dry off rather quickly.  And none of mine were at all fond of the process.
 
I am sure others have some ideas.  These worked well for me.
 
Shirley and the boating ferrets, Dunkin' and Journey
[Posted in FML issue 3833]