Many thanks to Kilchoan Early Bird for these pictures, taken yesterday. They show an object he found....
....washed up near Ardnamurchan Point lighthouse.
The markings on it clearly indicate that it's something used by the Canadian Coast Guard.
As can be seen from this map of the North Atlantic surface currents (courtesy 'Jack' via Wikimedia Commons), currents from the coast of Canada wouldn't have brought it to our shores, so it must have started from somewhere out in the Atlantic.
A search on the internet produces an object called a 'spar buoy' which looks like what Kilchoan Early Bird found - see here - but the reference doesn't really explain a spar buoy's function.