A solar cover is the only cheap option that works.

Yes, it’s the best way.

A solar cover—basically a big sheet of bubble wrap for your pool—floats on the surface and traps heat from the sun while cutting evaporation. Without a heater, you’re relying on the sun and ambient temperature. A cover can bump the water up 5–10 degrees over a few sunny days.

Black solar rings or hoses help a little, but they’re more work and less effective. A cover is cheap, easy, and pays for itself in one season.

Don’t bother with the DIY black hose spiral unless you enjoy spending weekends coiling plastic.