Ahoy,
I run into the same problems here in Florida. My duck run was barren of any trees when I fenced it in. Since then I've planted 3 Live Oak trees with more to be planted in the near future. The trees are still somewhat small, but Live Oak grow fairly quickly into decent shade trees. I intend to plant many more trees around the area I plan to dig a pond. I'm hoping that the less direct sunlight that hits the pond water, the less rapidly the algae will grow.
Next I have a 10'x10' pen with a solid Ondura roof, but open sides with 1/2 steel mesh to contain my Welsh and Cayugas. The roof provides the only significant shade the flock currently has, but the mesh sides give them plenty of air flow through their quack shack. The trees provide some shade, but until all 21 can fit in the shade at once, they prefer to relax in the shade of their house.
Rather than a black rubber trough for water, I use a galvanized steel one placed inside their pen in the shade. The metal allows the warm water to cool faster than rubber does. It's hooked up to an automatic waterer, so any refill comes in small amounts rather than filling it up with hot "hose-in-the-sun" water all at once.
In the middle of the Florida summer, I notice the ducks will find a spot in the shade, sleep, pant, and reduce any activity to nothing. They always have water accessible and a few times I have added duck health powders to it. So far I have had no heat casualties.
Good luck,
Rich