My root was spread out by the waters - A metaphor taken from a healthy tree growing beside a rivulet where there is plenty of water; which in consequence flourishes in all seasons; its leaf does not wither, nor its fruit fall off. See Psalm 1:3; Jeremiah 17:8.

root

Job 18:16 His roots shall be dried up beneath, and above shall his branch be cut off.

Psalm 1:3 And he shall be like a tree planted by the rivers of water, that brings forth his fruit in his season; his leaf also shall not wither...

Jeremiah 17:8 For he shall be as a tree planted by the waters, and that spreads out her roots by the river, and shall not see when heat comes...

Hosea 14:5-7 I will be as the dew to Israel: he shall grow as the lily, and cast forth his roots as Lebanon...

spread out. Heb. opened

The Case of the Christian under the Hiding of God's Face.
1. The phrase scriptural.--2. It signifies the withdrawing the tokens of the divine favor.--3 chiefly as to spiritual considerations.--4. This may become the case of any Christian.--5. and will be found a very sorrowful one.--6. The following directions, therefore, are given to those who suppose it to be their own: To inquire whether it be indeed a case of spiritual distress, or whether a disconsolate frame may not proceed from indisposition of body,--7. or difficulties as to worldly circumstances.--8, …
Philip Doddridge—The Rise and Progress of Religion in the Soul

No Sorrow Like Messiah's Sorrow
Is it nothing to you, all you who pass by? Behold, and see, if there be any sorrow like unto my sorrow! A lthough the Scriptures of the Old Testament, the law of Moses, the Psalms, and the Prophecies (Luke 24:44) , bear an harmonious testimony to MESSIAH ; it is not necessary to suppose that every single passage has an immediate and direct relation to Him. A method of exposition has frequently obtained [frequently been in vogue], of a fanciful and allegorical cast [contrivance], under the pretext …
John Newton—Messiah Vol. 1

Job
The book of Job is one of the great masterpieces of the world's literature, if not indeed the greatest. The author was a man of superb literary genius, and of rich, daring, and original mind. The problem with which he deals is one of inexhaustible interest, and his treatment of it is everywhere characterized by a psychological insight, an intellectual courage, and a fertility and brilliance of resource which are nothing less than astonishing. Opinion has been divided as to how the book should be …
John Edgar McFadyen—Introduction to the Old Testament