correct. It's generally not feasible to shade such a large area over water. But evaporation does take its toll on reservoir water. There was a proposal not long ago to shade the canal carrying water from Northern California to Southern California. Reducing evaporation was one of the touted benefits[1].

  A study by the University of California, Merced gives a boost to the idea, estimating that 63 billion gallons of 
  water could be saved by covering California’s 4,000 miles of canals with solar panels that could also generate 13 
  gigawatts of power. 

1 - https://www.pbs.org/newshour/economy/solar-panels-built-over...

Shading is used in California to prevent evaporation, see for example the well-documented use of "shade balls": https://en.wikipedia.org/wiki/Shade_balls

Their use in Los Angeles was covered in a Veritasium video that was all over YouTube for a while (97M views): https://www.youtube.com/watch?v=uxPdPpi5W4o

The estimate given when they were added in 2015 – in the middle of a record drought – was that they would prevent the loss of 300 million gallons of water each year.

Not even directly that I think, it's to prevent chemicals that are used to stop algae growth to interact with sun.