This is just fan fiction, it's not in Minecraft.
The sea serpent is an optional boss who must be fought to reach Minelantis .
In order to fight the Sea Serpent, you first must obtain the "magic conch". It is a shell that summons the serpent. To get this, make a 3x3 1-block deep hole (it has to be in the sand within 5 blocks of the ocean) and put a block of gravel in the middle. Put 1 block of sand on top of this and fill the hole with water. Use bone meal on the top sand block and the conch will pop out!
This is a cutscene boss. The first cutscene plays when you right-click the conch.
Player: Hmm, what's this? (Blows into it, it plays eerie music, but nothing else happens.) Player: Huh. I guess... Sea Serpent: (shriek from nowhere, then it crashes up onto the beach, sending the player into the water.)