Why did Hashem bring darkness?
Rashi: This was for a Sukah (cover and protection) - "the cloud and the darkness separated between [the camp of] Egypt and Yisrael" (Shemos 14:20).
Why does it mention "Chashras Mayim Avei Shechakim"?
Rashi: The darkness came from thick clouds that drip water on the ground.
Malbim (Tehilim 18:12): Hashem sent clouds to give rain to extinguish the fire. This led to a new affliction - lightning.
What is the meaning of "Chashras"?
Rashi #1: It is an expression of a sieve; [from such clouds] fine [drops of water] fall on the ground.
Rashi #2: It is an expression of being tied. Shamayim is tied with clouds via the water. "V'Chishureihem" (Melachim I, 7:33) said about the wheels of the Mechonah (laver), were wooden arms that tied the wheels together.
Why does it say here "Chashras" and in Tehilim 18:12, "Cheshchas"?
Radak: These are similar. When the clouds are tied to each other, there is darkness.
