What role could sea dragons have in my world?

Unless the serpents go beyond the sea and can be found in lakes, great lakes and the greatest lakes (and lochs) then it may be hard to get them into the overall story. It wouldn't be out of the question, seeing the mythological and folkloric versions appear in those sorts of places. Could also have them in underground lakes and rivers too. Just like seagulls, they aren't actually limited to the sea. They're just not as likely to be found in dumps.

 

I managed to slip a sea monster into my most recent fantasy novel (Tsar of the Empty Lands), what is essentially a remnant mosasaur that hides in deep fjords and hunts by ambush. I've no idea whether actual mosasaurs did that sort of thing but it seemed plausible enough. This makes the creature, which I dubbed the 'itzo,' the 'shadow of the deep places,' a threat to small boats along the coast so I didn't have to go out to sea any distance to incorporate the creature into a tale set mostly on land.

 

You might like something similar to the Behemoth. They dwell in deep rushing rivers though. They basically something like Argentinosaurus, but prefer to cool off in large bodies of water. There's Leviathan as well, they are commonly known as sea monsters. But they have a tendency to come to land. There are two types of Leviathan, the ancient description and the modern style leviathan. The ancient type is something similar to the Hydra, while the modern type has only one head.

 

In Maori culture there is a belief in the taniwha. These creatures live in deep pools in rivers, dark caves and at sea. Legends often speak of battles between warriors and taniwha both on land and at sea. They often patrol areas deemed to be tapu (sacred or off limits for some practical or spiritual reason) and will attack people who enter such places or cause a disaster such as an earthquake. Their exact appearance is a matter of debate as they are represented in different ways by different tribes or whoever is telling the story but they often resemble a shark, whale or serpent.

 

I would use them as chaos beasts, like tanninim in Hebrew mythology. There is also surprising variance in sea serpents, such as some with sealish heads and yellow polka dots.

 

Perhaps as guardians of certain islands. I’ve always liked the idea of being able to tame them or control them somehow, and use them as transport or to pull ships.