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?