Question

at which position does the diver have the most gravitational potential energy

Answer 1

Directly vertical. The surface area is the smallest, allowing for the water upthrust to be kept to a minimum, whilst the mass is the same, which keeps the same weight and gravitational pull.

Answer 2

Potantial energy formula = m.g.h
m: mass
g: gravitational acceleration
h: height

m and g are the same. So, we must to look at to the height. At the vertical position, the potantional energy increase. 

When the h increase, the energy will increases.