I have found that solar covers DO heat the water. I think it's a myth that they don't. I've had blue, opaque (black underneath, blue on top) and clear. I thin the blue ones don't heat too well but the opaque and clear ones do. The opaque heat by conduction--the water right under can get hot where it's next to the black (but why the blue is on top and the black down is a mystery). Clear and blue heat by letting the sunlight through--clear lets the most through.

I have a blue one now and it's far inferior at heating to its clear (but far more expensive) predecessor.

Of course I rely on solar panels primarily, and the cover is to hold the heat in.

The bubbles not only ensure the cover floats, they provide dead air space, which is the best insulator short of a vacuum.