you compress the spring at the top and use 1 unit if energy
and the box sinks.
then if you decompress the spring under water so that it will expand and float up , then you are making the water level at the top increase , and you are lifting the water using the 1 unit of energy you put in compressing the spring.

you still have the energy from buoyancy as it rises to the top.

so you can compress the spring again.

but your problem is that you are working against the water pressures at the bottom.

if you didnt have to expend the 1 unit of energy at the bottom pushing against the water pressure
you could compress the spring at the top again.

and if you let the box rise more you could get more energy out from buoyancy.

this would be your extra energy over 100% efficicency.

3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.