Doesn't necessarily require a ocean biome, maybe be near one but not actually need to be in one, have had a serpent spawn near me when I was just 20 meters from a shore. Granted I have also seen a turtle spawn literally around 30-40 meters from a shore also...coulda rafted to it but a troll was chasing me.
If you found 1 during the day then congratz, I never seen one during the day lol