A pool cover (a.k.a. a solar blanket) really has no solar properties. It does not increase the effects of the sun's rays, as multiple magnifying glasses. The bubbles are not lenses, they only act as floating devices for the plastic cover. The pool cover... covers your pool! It traps the heat in by reducing heat loss through evaporation, overnight when the cold air sucks the heat out of warm pool water. Your pool acts as a heat tank; your cover slows down the heat loss.
On average, a pool cover retains 6 to 8F. It happens to be your best investment for keeping your pool warm.
But if you dislike the pool cover routine, mornings and evenings, solar heating can let you discard it, at least from June 10 to August 15. Upon season start, and season end, a cover remains very efficient if your objective is to extend the season. Combined with solar heating, a cover will allow you to swim in relatively warm water, even when the air is cool.
The main advantage of solar heating being free heat from the sun, one can do without a cover altogether, and not worry about fuel costs by adding 50% more collectors.