I read it on a news group.  There is a very ferret knowledgeable person
who lives right near the ocean.  This topic came up and he shared his
knowledge about it.  It's been a while, so I don't recall the
particulars, but he strongly discourages letting ferrets swim in salt
water.  He cited a bunch of technical stuff that I don't understand, but
he seems to know a lot more about it than I do, so I take his word for
it.  He never lets his ferrets get in the ocean, only to the edge of it
(he takes his ferrets to the beach regularly).
 
E-mail me privately and I'll give you his name and you can discuss it
with him.
 
Gary
[Posted in FML issue 4340]