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

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.

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.