"Or who guides you in the darkness of the land and the sea"
This means: Who directs you during the darkness of the nights, on land and at sea, by means of the stars and similar signs. The attribution of the darkness to the land and the sea is due to their association and their presence within them. It is also permissible that the "darkness" refers metaphorically to confusing paths, as they are like darkness in causing bewilderment.
"And who sends the winds as glad tidings before His mercy?"
The interpretation of a sentence similar to this has already been provided.
"Is there a deity with Allah?"
This is a negation of there being any other god alongside Him, Glory be to Him.
"Exalted is Allah above what they associate with Him"
This is a confirmation and verification of the preceding, and the explicit mention of the Majestic Name [Allah] in the place of a pronoun is to indicate the reason for the judgment. That is: He is exalted and transcendent by His Essence, which is unique in divinity—a status that necessitates all attributes of perfection and qualities of majesty and beauty, which in turn necessitates that all created beings be subject to His power.
"Above what they associate" means: above the existence of that which they associate with Him, Glory be to Him, under the guise of it being a god or a partner to Him. Or, "Exalted is Allah above the partnership or comparison of what they associate with Him." It is also permissible for the word "ma" (what) to be an infinitive particle, meaning: Exalted is Allah above their act of associating partners. It is also recited as "‘amma tushrikūn" (above what you associate), based on the second-person address.