1.
He fixed darkness to be His veil;
It refers to trubles, which for them is darkness.
An alternative interpretation could be: When setting their destiny, God sets the darkness of concealment, canopy around Him, so no one could see Him, in other words hides Himself from them.
dark waters...
Here the word [heb. CHESHCHAT] - (Darkness) is used.
In the book Shmuel II 22:12, [heb. CHASHRAT] - (Gathering of waters) is used
Both cases have a common meaning, because accumulation of clouds at some point produces darkness from the gathering of waters.