This is a very good question, and a common one too. Many people wonder if they should place their pool blanket bubbles up or down. The answer is definitely bubbles must face down.
Why is this? Your pool cover is designed to absorb heat from the sun and then transfer this heat into the pool water. By facing the bubbles Downwards into the water, the sun’s heat is trapped in the blanket, and the air inside the bubbles is heated up and then is transferred, via the bubbles, into the pool water.
If you accidentally place your pool solar cover on the pool with the bubbles facing up the bubbles can be damaged. How do they get damaged? As they absorb heat, they won’t be able to transfer that heat into the water and will overheat. This will damage the material that the solar cover is made from.