Question

Why 90% of all marine life is found in the upper 180 meters of the ocean?

Answer 1

In lieu with your question, only 10% marine life are adapted to withstand the immense hydro-static pressure below 180 meters. Other 90% are adapted to live in surface of  ocean or on their evolutionary way to land.

Answer 2

Most fish need some form of sunlight and the other 10% found other way to survive with little to none