The sea had not come into existence only after the Sea God appeared. The sea had existed since the birth of Douluo Star.
Most life dwelled here. Since ancient times, the sea had followed an iron law of nature: big fish ate little fish, little fish ate shrimp, and shrimp ate plankton.
Later, Poseidon rose within the sea. He saw its potential and decided to rely on the sea to become a god.
Thus, he divided Sea Soul Beasts into two categories. Fierce predators were classified as Evil Soul Beasts, including Deep Sea Demon Whales, Evil Spirit Orcas, Giant Squids, and the like.
Meanwhile, those at the weak end of the oceanic food chain, the vulnerable groups often eaten by big fish, were classified by Poseidon as Good Soul Beasts.
Good Soul Beasts had an enormous population and could provide him with considerable faith to ascend to godhood.
So he protected Good Soul Beasts, protected Sea Soul Masters, and suppressed Evil Soul Beasts in exchange for the faith of Good Soul Beasts and Sea Soul Masters. After a thousand years of accumulation, he finally became a god in one stroke.
What kind of concept was that?
Imagine going to the African savanna and, relying on overwhelming force, saving gazelles, wildebeests, zebras, and other kind herbivores from the jaws of evil carnivores such as hyenas, lions, and leopards. You forbade them from harming these kind lives, protected them as they leisurely grew and reproduced across the savanna, and the kind herbivores, grateful for your protection, offered you their sincere faith. Thus, you became the God of the Savanna, the deity of that grassland.
Still, Poseidon was not quite that extreme. The sea also required sustainable development. Whenever Good Sea Soul Beasts had enjoyed peace for too long, forgotten the threats of