This helps control evaporation losses as well, the water flow in canals is more predictable which helps. Also there is a great potential for floating solar especially behind existing hydro as the grid infrastructure almost exists.
A good point from a dead comment :
"I wonder how this would affect the climate, the rains especially."
Reducing water evaporation should have some kind of effect. Does anybody know of weather and/or rain effects due to these evaporation preventing floating black balls on reservoirs?
Less evaporation losses means more water will reach irrigation networks where it will evaporate anyways (either before or after temporarily becoming part of a plant)
Given that a square of 300X300km would produce enough electricity to power the WHOLE WORLD, and 300X300km square of ocean is such a minuscule thing, I'd say -- no measurable impact.
