John Gill Commentary Isaiah 48:18

"Oh that thou hadst hearkened to my commandments! then had thy peace been as a river, and thy righteousness as the waves of the sea:" — Isaiah 48:18 (ASV)

O that you had listened to my commandments Which the Jews did not, but slighted and despised them, and were not obedient to them. So, in the times of Christ, they disregarded his doctrines, though so profitable; and despised his ordinances and commands, which were not grievous; they neither listened to them themselves, nor would suffer others; wherefore our Lord expresses his great concern at it, and his desire, as man, after their welfare; see (Matthew 23:13Matthew 23:37).

then had your peace been as a river : their prosperity, temporal and spiritual, had been abundant, and would have always continued, have been increasing and ever flowing, yea, overflowing, like the waters of a river. The Targum is, the river Euphrates, a river which ran through Babylon: but they had no regard to the things which related to their temporal, spiritual, and eternal peace, these were hid from their eyes, (Luke 19:42).

and your righteousness as the waves of the sea : large, abundant, numerous as the waves of the sea; which may regard acts of justice and righteousness, which are the support of a people and state, and blessings the fruit thereof; and which God of his goodness bestows on such a people, as all kind of prosperity, protection, safety, and continuance.