Covering one the New Zealand's hydro dams in panels would produce same amount as hydro dam itself. Floating solar is somewhat easier to install as you don't need to drive piles, you let them float on (anchoring is needed tho). Round platforms can somewhat easily rotate to track sun. You already have most of grid there and you can curtail hydro when sun is out...

Instead we burn diesel and coal to preserve water levels for peak demand (it's actually not that simple, but still sounds dramatic).