We have the formula for gravitational potential energy is,
E=-GMm/r
where, G is universal gravitational constant
M is the mass of earth
m is the mass of diver
r is the distance between diver and centre of earth
and negative sign(-) is used because work has to be done against the force of gravity.
At constant GMm, E is inversely proportional to the -r.
So when diver goes lower and lower, value of r decreases and makes the magnitude of E high. But the negative sign converts the value into least.
Therefore, diver has least gravitational potential energy.