Adam Clarke on Job 41:32

He maketh a path to shine after him - In certain states of the weather a rapid motion through the water disengages many sparks of phosphoric fire. I have seen this at sea; once particularly, on a fine clear night, with a good breeze, in a fast-sailing vessel, I leaned over the stern, and watched this phenomenon for hours. The wake of the vessel was like a stream of fire; millions of particles of fire were disengaged by the ship's swift motion through the water, nearly in the same way as by the electric cushion and cylinder; and all continued to be absorbed at a short distance from the vessel. Whether this phenomenon takes place in fresh water or in the Nile, I have had no opportunity of observing.

The deep to be hoary - By the frost and foam raised by the rapid passage of the animal through the water.