Why does the Torah write that "No'ach was six hundred years old when the Flood was water on the earth", and then "No'ach ... entered the ark before the water of the Flood", as if the flood preceded his entry into the ark?
Rashi: Precisely because it did. No'ach was lacking in faith; he did not believe that the Flood would come, and it was only when the flood-water began to fall that he actually entered the ark. 1
The Ramban, citing the Midrash, explains that it was only when the water reached his ankles (and it was clear that the people would all drown that he entered.
It should have said 'the water was a flood on the ground.' Why does it say "veha'Mabul was water on the ground"?
Moshav Zekenim: Even the trees and rocks were Novlim (withered), and became flood water.