In olden time the serpent returns to the ocean, they were land animal, but the biological tribulation drove to the ocean.
And they began to eat-up all the fish in the sea until they grew fat, over 40 feet long. But they have small brain.
So, one day when all was well, they had no fear in the ocean, they were untouchable.
Then sudden, the small fish began to attract them, and kill them and eat them.