Solar blankets are the best for evaporation and heat loss.

Bubble covers (solar blankets). They float on the surface, trap heat, and cut evaporation by 90% or more. Cheap, easy to cut to size, and they pay for themselves in water and heating savings in one season.

Automatic safety covers work too, but they’re mostly for keeping stuff out of the pool and preventing drowning. They don’t insulate as well because they sit on top of the water with a gap. Solid winter covers are useless for evaporation.

You don’t need anything fancy. A 12-mil blue bubble cover does the job. Thicker lasts longer but takes more effort to roll up. Don’t overthink it.