The UK has covered some water reservoir with floating PV panels.

The article mentioned trialling several systems for covering California canals, I didn't see "floating" mentioned, although:

  Project Nexus tests not one, but three different solar canal models. 

  ... accordion-like retractable panels are positioned low across the narrow canal, ready to roll up at a moment’s notice. 

Floating panels have the issue of having to flex and perhaps bottom out (if water diverts to parallel canal for maintenance) on a dry canal - but there's less need for heavy steel trusses and a lower wind profile.