at what depth does water stop looking darker from above?

[MysteriousWeeb420]

with every other factor the same surely water depth makes no difference

Cant we extrapolate that there should be a difference from comparing small depths? Take those first 20-30 m on the beach as an example; you can even spot the difference while standing in it

No, that's just because the deeper water has more impurities in it. They would both be crystal clear otherwise