The British temperature might've had something to do with that
In the 80's, i don't think ANY polar bear enclosure had temperature control, however in the modern day, I don't think it is as important for Polar Bears in the UK as it is on the Gold Coast in Australia or for Polar Bears in the hotter areas of the US (San Diego etc) to have cooled enclosures.
(And that water would be pretty chilly throughout most of the year)
The photo of the bear enclosure was not the original one for polar bears. In the late 60's CZ built 3 moated enclosures for bears. A workman was killed when working on the moat when a wall collapsed. These enclosures were on the site of the old barred cages and incorporated the inner quarters. This is the first enclosure of the 3 and initially showed sloth bears. The polar bears were moved there whilst their original enclosure was being revamped. The plan shows the sloth bears in this enclosure.
The animals in this photograph would be Bonnie & Clyde. I think they both died at Zagreb Zoo in their early 20s.
The bears would pace along the edge of the pool for much of the day, while dance music from the central rides and fairground played loudly.
Does anyone know what their accommodation was like at Zagreb zoo?