A few thoughts:

Get an opaque cover--and as thin a one as possible. The thicker covers (16mil) insulate better (good in my part of the country) so go for thin. Opaque covers don't seem to heat as well as transparent ones--again, that's experience.

The other thing that occurs is get solar panels--and only run them at night. They will act like car radiators to bleed off heat. Then, in the cooler months you can use them in the daytime to warm the pool.