Gatekeeper Puff is the one who opens the gate to the second sea. For him to open it you will have to give him a world scroll which can be found going a bit to the right of the north marker on the top compass in the rough seas to the ???? Shrine. When you have returned with the scroll he will say "I see you have been mentally farmed :)" and he will proceed to go into a cutscene and open the gate.
There is a bug where you cannot interact with him unless you rejoin.
You can talk to him once you reach level 325 if not, he will say "ur too weak to explore futher"