Final answer:
The mechanical energy of an object includes both its potential energy and its kinetic energy. In this case, when John is standing on the diving board, he has 200 Joules of potential energy.
Step-by-step explanation:
When John is standing on the diving board with 200 Joules of potential energy, this represents the mechanical energy he has due to his position above the ground. When he jumps and lands in the water, assuming no energy is lost due to air resistance or other non-conservative forces, his mechanical energy is conserved. Therefore, when John lands in the water, his mechanical energy remains 200 Joules, now entirely in the form of kinetic energy, since the potential energy becomes zero at the water level.