Can I warm up a swimming pool by submerging black-painted milk jugs in it?
I'm not clear on how some milk jugs are going to heat the pool. Most of the light will have scattered before it reaches the submerged jugs. Plus the surface area is going to be minuscule compared to the pool area, unless you have thousands of jugs.

Finally, I thought one of the main benefits to a pool cover was to reduce evaporative heat loss. Underwater jugs won't help you at all.