A floating spa or hot tub thermal blanket is great to use to keep your water warm and protect your solid cover from the water's chemicals. The cover will reduce water evaporation which will save the hot water and chemicals from leaving your hottub.
If your hot tub is not inside or under a patio, you'll need to pick up a spa umbrella. The umbrella stand slips under your hottub and you can swivel and tilt it to block the sun.