Who’s ass was so big, she couldn’t keep it completely go under water; it was keeping her buoyant, despite her best efforts.

Just for a refresher:

The Buoyant Force acting on a body is given by:

FB=γV, where

FB= Buoyant Force
γ= Specific Weight of the Fluid
V= Volume of the Body

Therefore, the buoyant force has a magnitude equal to the weight of the fluid displaced by the body and is directed vertically upward on the body.

From this, we can deduce that this woman’s ass was displacing more water than the ass itself weighed, which is moderately impressive.