The buoyancy is calculated based on the bounding box of the object ("Submergence" function).

		
			protected virtual void OnTriggerStay(Collider other)
{
    other.SendMessage("OnWaterStay", C, SendMessageOptions.DontRequireReceiver);
    R = other.GetComponent<Rigidbody>();
    if(R!=null)
    {
        //Flow
        R.velocity += globalFlow * Time.fixedDeltaTime / R.mass;
        //Drag
        V = R.velocity.magnitude;
        R.velocity = R.velocity.normalized * Mathf.Clamp(R.velocity.magnitude - ((density * V * V) / 2 * Time.fixedDeltaTime * R.drag) / R.mass, 0, float.PositiveInfinity);
        //Angular Drag
        V = R.angularVelocity.magnitude;
        R.angularVelocity = R.angularVelocity.normalized * Mathf.Clamp(R.angularVelocity.magnitude - ((density * V * V) / 2 * Time.fixedDeltaTime * R.angularDrag) / R.mass, 0, float.PositiveInfinity);
        //Buoyancy
        R.velocity -= (Physics.gravity * density * Submergence(C.bounds, other.bounds) * Time.fixedDeltaTime) / R.mass;
    }
}

public static float Submergence1D(float C,float R,float c,float r)
{
    if(R<r)
    {
        var tmp = R;
        R = r;
        r = tmp;
        tmp = C;
        C = c;
        c = tmp;
    }
    return Mathf.Clamp(2 * R - (Mathf.Abs((C + R) - (c + r)) + Mathf.Abs((C - R) - (c - r))), 0, 2 * r);
}

public static float Submergence(Bounds W,Bounds O)
{
    return Submergence1D(W.center.x, W.extents.x, O.center.x, O.extents.x) * Submergence1D(W.center.y, W.extents.y, O.center.y, O.extents.y) * Submergence1D(W.center.z, W.extents.z, O.center.z, O.extents.z);
}