Genesis 7:18

And the waters waxed mighty and increased greatly upon the earth, and the ark went floating upon the face of the waters.

AND THE WATERS PREVAILED (‘Vayigb’ru’). AND THE WATERS ‘GAVRU’ (PREVAILED). This means that they increased exceedingly, for the Hebrew language calls great abundance gvurah (strength, power). So too, And their transgressions which ‘yithgavru’ (have prevailed), meaning increased exceedingly; His mercy ‘gavar’ (has prevailed) toward them that fear Him, meaning increased. And so also: If ‘bigvuroth’ four-score years,, 90:10. meaning with great abundance.
It is conceivable that the meaning of vayigb’ru is that the rains came in a rushing downpour, uprooting trees and toppling buildings, since power is called in Hebrew gvurah (strength) because strength lies in power. In a similar sense are the verses: They also ‘gavru’ (wax mighty) in power; ‘Vehigbir brith’ (And he shall make a strong covenant) with many for one week, meaning he will establish the covenant with firmness. And in the words of the Sages, [we find the expression], gvuroth geshamim (the powers of the rains), because they come down with strength. It is possible that the verse, If ‘bigvuroth’ four-score years,, 90:10. is of the same sense, i.e., if his bones and body be strong, and he is a man of power, he will live four-score years. And if so, ‘gavru’ upon the earth will mean that the waters were in their complete strength, overcoming even the high mountains and inundating them.

ותלך התבה, not under its own power but by the pressure of the external waters.

ויגברו המים, this verse adds a new dimension to what we have been told already, i.e. that the waters became turbulent. This is why the Torah adds the word מאד מאד twice, something most unusual. The repeated statement וירבו, that the waters still kept increasing in volume, is meant to tell us that the waters did not only increase in turbulence but also kept increasing in quantity. Whereas originally, the waters could only raise the ark, now, thanks to the turbulence, the ark was driven in different directions from where it had stood originally.