I wonder if a cover needs to "breathe" in some way? I've got a new Jacuzzi J-470 and the cover leaks when it rains. So, Jacuzzi is sending me a new one. In the meantime, I'm throwing a plastic (albeit not vinyl) tarp over the cover when I know it's going to rain. I've noticed when I do, the water temperature runs hotter by several degrees than my set temperature (which shuts my circ pump off). When the plastic tarp is off, it all runs normally -- stays at the set temperature, and the circ pump runs when it should.
On the other hand, here in the mid-Atlantic right now, the ambient outdoor temperature is in the high 50's. Come the dead of winter, maybe the above results would be different?