Ecological studies have to be performed very well. There are not sufficient studies in that respect. 

For instance, effects on fish population and fish health, effects on water plants, effects on water quality in different depths. 

To my mind nobody knows those kinds of things yet because the technology is not so mature yet. 

I highly recommend researchers who study ecological subjects to focus on those topics. 

Similarly, the microclimatic effects are also still unknown.

I highly recommend researchers who study climatic subjects to focus on those topics. 

Moreover, plastics and toxic PV materials have to be taken into account during design studies. In stead of plastic materials in floats, pontoons etc. natural materials must be used. For instance, hemp, bamboo. 

That part is a research gap. Researchers in related research fields have to focus on these issues.

For toxic PV materials. Some module materials are toxic some not as far as we know in state-of-industry. None toxic ones have to be selected in the design process. 

Moreover, demolishing and recycling have to be studied in advance. 

These are the downsides and research gaps that I know. 

The evaporation will be less when there is floating PV power plants on surface water bodies. 

Hope I can give some brief explanation and idea.