"I understand that in Scandanavian countries, plunges into ice cold pools and streams are
believed to be highly beneficial to health. A dog that enjoyed such hypothetically healthful
plunges would be a corker!"

Dear WW: In Scandinavia and many other places, a sauna, hot steam bath precedes the cold dip.
I saw on Internet a site about a place on North Sea English coast of a place called Scara Brae, where they have found a sauna used in 8000 BC. They heated rocks in fire, and threw them into water to make steam in an enclosed space before skinny dipping in ocean. It is said to be a very pleasant experience. I doubt my cardiovascular system could tolerate it.

All Newfoundland dogs joyously go into winter ocean to rescue people.. And the St.Bernards were born with a cask of schnapps lsuspended from their collar to help revive victims they rescued in the Alps.