After helping Jack of Alltrades, you should go south to find out what is happening in Porth Llaffan.
Visit Porth Llaffan
Porth Llaffan is a fishing village that had fallen on hard times since the fish stopped coming in after the earthquake. However, ever since Jona's father Dylan was lost at sea during the earthquake, Jona is somehow able to summon the Lleviathan to give everyone fish. But a few people say that all is not well. After witnessing this miracle, Jona goes to the Mayor's house.
Be sure to read the books in the Mayor's house to learn some new recipes. Then at night time, go to Jona's house (the easternmost house in town) and talk to her. Then go outside and talk to the ghost on the shore. Next, go to the mayor's house. There will be a cutscene. Now you can talk to the mayor's son who is standing by the gate at the northwest side of town.
When you go through the gate, follow the path west to Tywll Cave.