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This is a submission for the Anatomically Correct Series.
Sea serpents appear in various mythologies. Mesopotamians have Tiamat, Greeks have Hydras,Jews have Leviathan, the Norse have Jormungandr.What they all have in common are that there these giant snake-like creatures under the sea that can easily capsize contemporary ships.
Sea Snakes are real life animals but Sea Snakes aren't any larger than land snakes. Is it biologically possible for a whale-sized snake to evolve?
May I present the Oar Fish
I don't know much about them, but HERE is the wikipedia link to the most real sea serpent I have ever heard of.
The titanboa, the largest known snake, offers a look into what would be needed to make bigger snakes. It existed in a considerably warmer climate, but it's going to be hard to get that in an ocean.
The first thing, therefore, is a higher metabolic rate. Snakes are cold blooded, and limited in growth by this - I'd imagine there's a limit to how large a snake a cold blooded metabolism could support - at a point, the relatively sluggish digestive system would be a limiting factor
So, making them warm blooded would remove this limit, and, as such, the size limitation - no reason a sea snake can't be as large as a whale shark in this case.
It's obviously not the only limit, but I'd imagine it's a big part of the problem - a warm blooded sea serpent is going to be able to chase down prey, and move faster when it comes to capsizing boats, too. A warm blooded metabolism is also cheaper to maintain the larger the creature gets - the square/cube law working in our favour, for once!
I can't see any other reasons that sea snakes couldn't evolve to enormous size - there's no limits on their internal organs being crushed by their own bulk, or of having to develop massively stronger skeletons.
Having something similar to a sea snake evolve to fill the same niche as baleen whales do on earth can explain the size and shape. sea snake's anguilliform swimming style is extremley efficient(source: https://journals.biologists.com/jeb/article/208/7/1329/16002/Eel-migration-to-the-Sargasso-remarkably-high). Assuming sea serpents wouldn't have any predators, they might keep getting longer throughout their whole life as lengthening their body means they can push off of more water at once. Territoriality could explain their tendency to attack ships, though I don't know what could cause them to be territorial.
