I have heard that in Florida there is quite an industry which grows house plants to sell. Most houseplants live naturally in the shade of the huge trees of the rain forest, so in Florida they build 'slat houses' to provide the needed partial shade.

How about putting a slat house of some kind over your pool to keep the sun off, rather than trying to cool the water once the sun has heated it?

You could add plastic leaves and flowers to the slats to provide tropical ambiance. (Real vines would be nicer, but they would drop leaves and bugs into the water.)