I had pool "wall foam" installed when our pool was built last summer (also lined the floor) in hopes of maintaining as much of the limited solar heat available to us. It came in a kit of a roll of 1/4" thick closed cell foam and some spray glue (no glue for the floor kit) and wasn't very expensive as I recall. I bought it from a pool supply website - I don't know if available in Canada.

I can't say how effective it is at retaining heat (having nothing to compare), but, every little bit helps. It does provide a soft feel to the walls. The floor foam has a couple problems for me - it doesn't make the floor soft as the water is crushing the foam and the seams in the floor foam under the liner collect dirt in a very obvious straight line. If I do it again, I'll probably drop the floor foam.

The most effective thing you can do to keep your precious (and expensive) heat is to use a thermal cover. It will not only slow heat loss due to conduction but also due to radiation and, most importantly, evaporation which can be tremendous on a cool, dry breezy night.