How can I calculate how much of an object has entered a trigger collider?

Hi all,

Ok, so I'm attempting to write a buoyancy script, and I'm trying to add functionality where the volume of the fraction of the object that entered the water will vary the force of buoyancy on that object. For example, if only the tip of the object enters the water, the force of buoyancy on it is very weak. However, as it continues to fall, the fraction of that object in the water increases, which I want in turn, to increase the force of buoyancy on it. And so I ask the question above.

By the way, I'm using Unity's physics for all these forces (only one actually, apart from gravity).

Thanks in advance,

adgeofgalaxy