Ferrets generally don't have inhalation allergies, and in animals that do,
like dogs, the signs are itchiness of the skin of the face and feet, not
watery eyes -
 
I would look closely at the environment - is it dusty at all?  Have you
washed their bedding since wintertime?  Always make sure in such cases
that their environment is dust and dirt free.
 
With kindest regards,
Bruce Williams, DVM
[Posted in FML issue 3776]