1. Why is a submersible needed to descend to the bottom parts of the ocean?

A submersible is referred to as watercraft. It's small in nature and it has been designed to operate underwater.

it has a pressurized hull which enables the submersible to withstand high pressure from water outside, the pressure outside is greater than the pressure inside. so in other words the submersible is needed to descend to the bottom part of the ocean because the submersible has been built to withstand a very high water pressure in the deep ocean depth.