"Coronavirus, a Venezia l'acqua nei canali torna limpida: "Meno traffico, meno scarichi""For a dramatic and fascinating comparison of Venice before and during the pandemic, watch this 1-minute gif from the BBC. No discussion of the water, but the difference in crowding is startling.
"...good news comes from Venice, where the water of the canals, usually cloudy, appears limpid and transparent, so much so that fish and the seabed can be seen..."
Addendum: This is also good - rolling live-stream cameras showing virtually empty streets, plazas, and canals. Hat tip to an anonymous reader.