The parallelism of the two clauses is probably one of contrast. If so, the proverb is a comparison between all teaching from without and that of th…

The words of a man’s mouth are as deep waters—that is, the words of a “man,” properly so called, are as deep wat…

The words of a man's mouth [are as] deep waters
The words of a great and mighty man; of an excellent and valuable ma…

The wellspring of wisdom in the heart of a believer continually supplies words of wisdom.