Sforno on Genesis 7:24/ספורנו על בראשית ז׳:כ״ד

1ויגברו...חמישים ומאת יום. These 150 days are counted from the beginning of the 40 days of rainfall. The level of water on the surface of the earth kept increasing for 150 days, concluding on the 17th of the seventh month (Nissan). On that date the ark came to rest (on the waters) seeing there was no pressure exerted on it from any direction. The pressure on the ark to move was exerted from the first day of the rain when the Torah simultaneously reported the fountains below opening and adding their water to that of the rain. (verse 11). The ark resisted this pressure until the waters on earth were so high that they lifted the ark from its base. When the days of rainfall had concluded, the pressure from the subterranean waters kept up until the end of 150 days. When that pressure ceased the ark finally came to rest on Mount Ararat.