1. cut the sphere into 2 and u see a circle.
2. put that circle on a graph paper, such that the y-axis is the depth of water, and the x-axis is the radius of the cross section.
3. you end up with a circle of origin (0,ro)
4. the general formula of a circle is (x - a)2 + (y - b)2 = R2 where (a,b) is the centre and R is the radius of the circle.
5. Hence, I use the formula (h - ro)2 + r2 = ro2