The Cattle - 6:63
"Say: "Who can deliver you from the darkness of the land and the sea when you cry out to Him, secretly and openly, ´If You deliver us from this, we shall be of the grateful ones´?"
The darkness of the sea:
The Light/24:40
Or it is like the darkness in the midst of a vast sea covered by wave after wave with a dense mist. Darkness piled on top of each other. When a man stretches out his hand, he can hardly see it. He whom Allah has deprived of light has no light at all.