If you were to use a fairly clear (at least to visible and infrared light) pool cover, then you would have the entire area of your pool heating the water and eliminating heat loss from evaporation. As you can see from calculations in this thread, in a white plaster pool around 60% of the sunlight gets absorbed by the water and heats it. In a darker pool, even more gets absorbed. If the average depth is 4.5 feet, then this translates to peak noontime heating of around 0.7ºF per hour. If there were no pool cover, then evaporation can significantly cool the water where 1/4" of evaporation would cool that same 4.5 foot average depth pool by about 5ºF (per day, if evaporation is 1/4" per day).

Generally speaking, use of a fairly clear solar cover is the most economical way to raise the temperature of pool water. The downside is that most solar covers that are clear also let UV light through so you don't get much benefit of reduced chlorine usage. With a darker cover, less UV gets through and one gets a lower chlorine loss as a result. Getting a dark mostly opaque cover will block UV and absorb sunlight that can heat the surface of the water, but you have to have good surface circulation to remove that heat from the surface efficiently -- it's not nearly as good at heating the pool compared to a clear cover. At the other extreme, a white cover would reflect most sunlight and prevent heating of the pool which would be useful in very hot desert climates in order to prevent evaporation (i.e. save water), reduce chlorine usage, yet not get the pool too hot.