There are products on ther market that claim to spread a thin layer of proprietary transparent liquid over the surface of your pool to reduce evaporation and thus retain heat.
One product is Solarpills (a.k.a. Solarballs, but not to be confused with the water purifier) work? The vendor claims that one ball helps retain heat for 12,000 gallons, works all day, and will last for one month. Solarpills were manufactured by SmartPool, but are no longer mentioned on their list of products.
Another similar product is Heatsavr:
Energy savings of up to 50%
Reduces water and chemicals lost via evaporation
Helps to lower your carbon emissions
Biodegradable
Non toxic – Independently safety tested
Tasteless, odourless and undetectable by pool users