As the graphics hint, wind generators would be a good fit too; presumably the utilities own some of the land surrounding a dam, and these are often wide open spaces for good wind velocity.
This is a fantastic idea, assuming that the ecosystem within the reservoir isn't adversely affected. The problem of dry times seems less important to me - wouldn't the 'rafts' just sit on the dry banks and continue to give power until the water filled again? If the topology of the flooded valley was an issue, could the rafts be strung together so that they hung over the water at low times? Given the amount of wide open surface that reservoirs have, this has to be a great choice to maximise the output from something that may have cost a great deal in terms of displaced homes and real estate.
These rafts could be used to replenish reservoirs with downstream water in dry times. That way,the dams could continue to supply power,although at a reduced rate. There are already power storage systems that use excess power to pump water to elevated reservoirs to supply peak power demands.
Not mentioned in this short article is the reduced evaporation from reservoirs which is a big issue for reservoirs in arid climates such as the US Southwest. Also notable is the cooling effect on the PV panels which provides a marked increase in power production. NREL has been pursuing this for some time: https://www.nrel.gov/news/press/2018/nrel-details-great-potential-for-floating-pv-systems.html
In Brazil, a floating unit with 11,000 m² has been in operation since 2019, in the Sobradinho dam reservoir. https://www.portalsolar.com.br/blog-solar/energia-solar/brasil-ganha-primeira-usina-solar-flutuante-no-reservatorio-de-sobradinho-ba.html
Nelson Hyde Chick
The solar arrays would also reduce evaporation of the Dam's water.
Another consideration is that these would also reduce valuable water evaporation.
Aside from providing electric solar power, and perhaps lessening the evaporation of precious water as well, could these floating units be retrofitted, with small windmills to make more electrical power? I think that there is a type of ocean buoy, used to provide electric power for deep sea research depots, with that feature already. If mounting heavy electric generators into the small windmills, would not work in this instance, the mini-windmills could empower lightweight air pumps, to force air through a network of hoses, to a collectively-shared big turbine outside of the dam reservoir, to propel a big heavy generator there.
i believe solar panels would also cool the water in the reservoir...might be good for fish!
I like the idea of floating PV rafts, during the heat of the day the reservoir water may mediate the PV efficiency reduction, and as said in many posts, even reduce evaporation over one portion of the reservoir. But during the night they are useless and the dam must provide for demand. Are engineers considering limited refill of the reservoir during high PV production?

I know that caliber of pumping requires huge capital outlay. Most dams are built while bypass channels carry the water - could those bypass channels be retrofitted and used as part of the refill process? These types of "mechanical" batteries are brilliant, but would they be practical? At least the rafts are and the increased MW production during the heat of the day would be welcome.

Fascinating article. Good writing Nick.