Looking at the incredible variety of marine life and how they've adapted to such hostile environments, you truly realize that truth is greater than fiction. This is especially seen in three areas of marine life marine mammals, deep sea life and marine life around deep sea hydrothermal vents. In each case, these remarkable creatures saw a niche to exploit in order to survive and they adapted to do just that.
The theory of evolution is itself evolving, like everything else. It used to state that all life on the land "progressed" from life in the sea. However, in the case of marine mammals, they clearly found the land wanting and made the sea their home. Even today, there are animals that are still mobile on land but more at home in the water include the hippopotamus and the otter. Perhaps in a hundred thousand years, they will more resemble dolphins and seals than hippos or otters.
There are a lot of fossils of long extinct species that exhibit the characteristics of both land and sea mammals, like the hippo and the otter. One of these is called Ambulocetus, which strongly resembled a furry crocodile. It had very short legs, a long, low-set powerful body made for swimming rather than getting about on land. The skull is the most remarkable feature; the nostrils were located on the top of the head, just like in today's whales.
Living in the deepest part of the sees, where sunlight can't even penetrate, is a wide variety of marine life. How did they adapt to life in complete blackness? Although most of their lives are still a mystery, part of their communications with other members of their species is with phosphorescence. Their bodies flash patterns to either ward off predators, call members of the opposite sex hither and who knows what else. This personal pack of lights has helped deep sea marine life like deep sea jellyfish to thrive in an area of low competition for food.
Arguably, the most amazing cases of adaptation are those marine creatures that live in the presence of hydrothermal vents. Unlike most of life on Earth, hydrothermal vent marine creatures do not need any sort of sunlight in order to survive not even to collect food dropped from above as with the deep sea marine life. They are reliant on chemicals from the vents to act as the start of the food chain.
On land, plants are eaten by plant eaters and then meat eaters. In the unearthly world of the hydrothermal vents, bacteria eat the chemicals from the vents. The vents constantly spew super-hot material from deep within the earth, most likely in the way volcanoes do. Small marine creatures then eat the bacteria. Slightly larger creatures eat the bacteria-eaters and so on.
Every year seems to bring new surprises about how life has evolved in order to survive in unimaginable places. How they managed to do so was through evolution and they are not done evolving yet.