Genesis 8:3

New English Translation

³The waters kept receding steadily^[a] from the earth, so that they^[b] had gone down^[c] by the end of the 150 days.

Genesis 8:3 tn The construction combines a Qal preterite from שׁוּב (shuv) with its infinitive absolute to indicate continuous action. The infinitive absolute from הָלָךְ (halakh) is included for emphasis: “the waters returned…going and returning.”
Genesis 8:3 tn Heb “the waters.” The pronoun (“they”) has been employed in the translation for stylistic reasons.
Genesis 8:3 tn The vav (ו) consecutive with the preterite here describes the consequence of the preceding action.

New English Translation (NET)