| Barnes' Notes on the Bible The contest between reticence on the one side and pertinacity in search on the other is represented as by a parable. The well may be very deep (compare the marginal reference), but the man of understanding" has enough skill to draw up the water even to the last drop. Every question is, as it were, a turning of the windlass.
Clarke's Commentary on the BibleCounsel in the heart of man - Men of the deepest and most comprehensive minds are rarely apt, unsolicited, to join in any discourse, in which they might appear even to the greatest advantage; but a man of understanding will elicit this, by questions framed for the purpose, and thus pump up the salubrious waters from the deep and capacious well. The metaphor is fine and expressive.
Gill's Exposition of the Entire BibleCounsel in the heart of man is like deep water,.... Pure and undisturbed, but secret, hidden, and hard to be come at: such are the things of the spirit of a man, the thoughts of his mind, the devices of his heart; which, though easily known by the searcher of hearts, are not easily penetrated into by men; or it is not easily got out of them what is in them, especially in some men, who are very close and reserved. This is true of wicked men, who seek sleep to hide their counsel; and of good men, especially studious men, who have got a great deal of wisdom and knowledge in them, but not very communicative, being slow of speech, and silent in conversation; but a man of understanding will draw it out; he will find ways and means to discover the secret designs of wicked men, whether against church or state; and, by asking proper questions, an understanding man will get out useful things from men of knowledge, the most reserved: some men must be pumped, and a good deal of pains must be taken with them, to get out anything of them, as in getting water out of a deep well, and which when got is very good; and so is that wisdom and knowledge which is gotten by an inquisitive man from another of superior knowledge, but not very diffusive of it.
Keil and Delitzsch Biblical Commentary on the Old Testament5 The purpose in the heart of a man is deep water; But a man of understanding draweth it out. "Still waters are deep." Like such deep waters (Proverbs 18:4) is that which a man hath secretly (Isaiah 29:15) planned in his heart. He keeps it secret, conceals it carefully, craftily misleads those who seek to draw it out; but the man of תּבוּנה, i.e., one who possesses the right criteria for distinguishing between good and bad, true and false, and at the same time has the capacity to look through men and things, draws out (the Venet. well, ἀνέλξει) the secret עצה, for he penetrates to the bottom of the deep water. Such an one does not deceive himself with men, he knows how to estimate their conduct according to its last underlying motive and aim; and if the purpose is one that is pernicious to him, he meets it in the process of realization. What is here said is applicable not only to the subtle statesman and the general, but also the pragmatical historian and the expositor, as, e.g., of a poem such as the book of Job, the idea of which lies like a pearl at the bottom of deep water.
Geneva Study BibleCounsel in the heart of {c} man is like deep water; but a man of understanding will draw it out. (c) It is hard to find out: for it is as deep waters, whose bottom cannot be found: yet the wise man will know a man either by his words or manners.
Wesley's Notes 20:5 Counsel - Designs of doing something of moment. Deep water - Is secret and hard to be discovered.
Jamieson-Fausset-Brown Bible Commentary5. Counsel . water-that is, deeply hidden (Pr 18:4; Ps 13:2). The wise can discern well.
Matthew Henry's Concise Commentary20:5. Though many capable of giving wise counsel are silent, yet something may be drawn from them, which will reward those who obtain it. 6. It is hard to find those that have done, and will do more good than they speak, or care to hear spoken of.
Matthew Henry's Whole Bible CommentaryVerse 5 A man's wisdom is here said to be of use to him for the pumping of other people, and diving into them, 1. To get the knowledge of them. Though men's counsels and designs are ever so carefully concealed by them, so that they are as deep water which one cannot fathom, yet there are those who by sly insinuations, and questions that seem foreign, will get out of them both what they have done and what they intend to do. Those therefore who would keep counsel must not only put on resolution, but stand upon their guard. 2. To get knowledge by them. Some are very able and fit to give counsel, having an excellent faculty of cleaving a hair, hitting the joint of a difficulty, and advising pertinently, but they are modest, and reserved, and not communicative; they have a great deal in them, but it is loth to come out. In such a case a man of understanding will draw it out, as wine out of a vessel. We lose the benefit we might have by the conversation of wise men for want of the art of being inquisitive. | 
