In Yu-gi-oh every serpentine-like or reptilian-like sea predator is considered sea serpent. Orcas have serpentine bodies. Also eels which are fishes and not reptiles like Kairyu-shin are sea serpents.
For me LGPE like you can see in the picture, because the difference of Battle System and the presence of only 1st gen pokemons made it worst than OR/AS, which at least had good pkmns like new megas and archeoforms and traditional battle s.