How does water density affect marine life?
The denser the water, the less light reaching those animals living deeper in the ocean, causing different specializations to adapt to that.
The denser the water the less light that can get through to the bottom of the ocean. Fish also have to adapt to this in order to survive and hunt their prey. Some have evolved huge eyes that can see better in the dark such as the giant squid. Some have evolved eyes to look upward, in order to better see the shadows of their prey from below. One of the coolest adaptations is called bioluminescence, which is the production and emission of light by a living organism. Some use it to lure prey in (angler fish) and some use it to communicate.
The sun is the primary generator of energy on planet earth, and everything that most animals eat comes from its energy. At the bottom of the ocean, where there is no light, some amazing animals and corals have evolved to survive and create entire communities where they don't need sunlight at all to survive.