Asperognathus taniwha

There is little could tell you about the live of adult Asperognathus, sometimes also called "sperm whale dragon". These animals, which live in the northern pacific, are very well divers, they often reach the light-less deeps under 1000m and adult males never leave the open sea. They survive in this deeps because of, heavy bones, relatively small lungs which compress during diving and a high portion of myglobin in their blood. They can dive up to 110 mintues.
Asperognathus hunts mostly squids, crabs and shellfish and sometimes they come together at carcasses of whales (most of what is written here is speculation based on the stomach content of hunted specimens). The best place to watch these middle sized sea serpents is the Lousville seamount chain, where they find enough food for a more dense, sedentary, population than in other regions of the pacific. One population in the middle of their distribution area is instead nomadic and migrate into the antarctic ocean during the summer (some authors suggest that these are two species or subspecies, others are sure that these are just eco-types) 

To the end of the antarctic summer the females swim back to the places of their birth, the sea-grass beds at the coasts of New Zealand and Chile, where they give birth to two, 70 cm long, babies.
The mother shortly swims back into the open ocean, but the juveniles remain near the coast until they can follow their parents into the dark deeps of the pacific. 

This species, especially the juveniles, found their way into the mythology of the Maori as part of the taniwha myth. 
In historic times Asperognathus was a very common sea serpent, but the intense fishery around New Zealand and the shrinking sea-grass beds are a problem which has already an not ignorable impact on the species.

Edit: Note the small pink bubble in front of the eyes is a salt gland, this, together with the fenestra behind it, are important automorphies of natrixosaurs.

Edit2: when you look closely you may notice barnacles on the skin...

Other sae serpents:
extinct freshwater sea serpents: hyrotrioskjan.deviantart.com/a…
Cavinatrix: hyrotrioskjan.deviantart.com/a…
Gracilinatrix: hyrotrioskjan.deviantart.com/a…
Bibonatrix: hyrotrioskjan.deviantart.com/a…