They use other sources of energy. Like geothermal waters, or just food.
Fish adapt to the pressure.
They live deep deep under alot of water, at rock bottom, were the water is not blue it is a dark black color.
Many species of sightless creatures live deep in caves.
Plankton ,Dragon fish ,the deep sea angler ,and the giant squid(octipus).
They need to adapt to high pressure in the (low part of the ocean) deep ocean.
It is important for organisms that live in the deep ocean to absorb heat, since it is very cold in that region of the ocean. Without the heat, they would die. Some organisms absorb heat from ocean vents, which provide a large amount of heat.
To allow all creatures to live free without harm.
They have adaptions to live there. Some have big ears to release alot of heat like the fennic fox. Some can store water easily in their body that lasts for months like the tortoise can store water under it's shell. Alot of animals dig deep burrows to escape the heat. Alot of the animals are nocturnal.
Because the snake can just get the creatures without any problems
Not generally. They are deep sea creatures, and do not come so close to shore, usually.
It is simple, if you live in a place where all you can get is fruit would you eat it or die? Since the food source is different down deep in the ocean then on the surface the deep sea creatures must eat what is available.
Yes there are many unidentified creatures in the sea. There are parts of the sea that are so dark that only creatures with special adaptations can live there. There are also parts of the sea that are so deep that we cannot go there. The pressure collapses submarines.