I wonder if floating PV would be an interesting alternative to the shade balls used in some reservoirs[1]? You could get the benefit of lowered evaporation and some extra pv real-estate. Maybe the math is favorable, maybe not.
"It was a desire to shade the recycled wastewater, not capture the sun’s energy, that first led city officials down the road to the solar installation, Crowley said. The city hoped to prevent algae from blooming in the two ponds — which hold treated water from the city’s municipal sewer system."
They were going to pay to cover the reservoir, and instead they get paid for the solar output.
You could site solar in a vineyard, though, and get both better crop yield and better efficiency. But that is not a reason not to put solar on your reservoir, too.
Canals and reservoirs seem like perfect places to shade with solar.