What does the Torah mean when it writes that "the water was strong on the earth (va'Yigberu ha'Mayim)"?
Ramban #1: It means that the water increased immensely. 1
Ramban #2: It means literally "forceful" (turbulent), 2 inasmuch as the torrent of water was so powerful that it tore out trees and toppled buildings, even on top of the tallest mountain.
Ramban cites a number of instances where Gevurah is synonymous with a large increase.
Similarly, he cites examples where the same word means, "with power." Indeed, Chazal (Ta'anis 2a) refer to rain as "Gevuras Geshamim."
According to the Seforno, the turbulence was caused by the waters of the Great Deep when they burst open (Refer to 7:24:1:1).
This also fits well with the rest of the Pasuk.
Rashi writes: "'va'Yigberu ha'Mayim' - 'The waters increased' - of their own accord." What does Rashi mean?
Gur Aryeh: The 40 days in which the waters increased [due to rain] had already passed (in verse 7:17). The word "va'Yigberu" must therefore mean that the waters began to increase by themselves. 1
Meaning, the waters of the depths rose up, as Mizrachi explains.
Why did the Ark not sink under the combined weight of the people and animals and the food?
Ramban (to 8:5): It was due to the velocity of the boiling water that came gushing from the depths (underground springs). 1
Sure enough, the moment the water ceased to flow, and the heat cooled down, the Ark rested on Mount Ararat.
According to physics, the Ark would float if its total weight was less than that of water of its volume, about 50 million kilos. If it was submerged 11 Amos, it weighed about 20 million kilos (PF).
רש"י: ויגברו- מאיליהן: למה פירש רש"י כך?