This would certainly be the cheapest way to get the pool temperature down a few degrees.
The "duckweed" must be something like cheap mylar plastic disks that spread out by themselves with very little clumping. Maybe surface or magnetic effects + slight wave action will work.
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The disks must either float just below the surface or wick water through for evaporative cooling. The disks should be large enough to not pass through the filtration / chemical system.
The disks also need to be non toxic.
Bret Cahill