Genesis 7:18

And the waters prevailed, and were increased greatly upon the earth,.... Still they became greater and more powerful, as to bear up the ark, so to cast down houses, trees, c. by the continual rains that fell, though perhaps they were not so violent as before, and by the constant eruptions of water out of the earth:

and the ark went upon the face of the waters it floated about upon them, in an easy gentle manner; for there were no storms of wind or tempests raised, which might endanger it.

(If much of the water came from volcanic activity, and if earthquakes accompanied the breaking forth of the fountains of the deep, many tidal waves would result. This would completely destroy and remains of the old civilisation and as well give the ark a rough sea to drift in. The ark's dimensions would give make it almost impossible to upset. Ed.)