Adam Clarke on Psalms 18:11

He made darkness his secret place - God is represented as dwelling in the thick darkness, Deu 4:11; Psa 97:2. This representation in the place before us is peculiarly proper; as thick heavy clouds deeply charged, and with lowering aspects, are always the forerunners and attendants of a tempest, and greatly heighten the horrors of the appearance: and the representation of them, spread about the Almighty as a tent, is truly grand and poetic.

Dark waters - The vapors strongly condensed into clouds; which, by the stroke of the lightning, are about to be precipitated in torrents of rain. See the next verse.