By answering three little quizzes from the waitress in the Rusty Rudders, you will have a chance to access a cannon and blow an opening to a cave entrance. Inside the cave is a Hydroculus so you might want to solve this puzzle ASAP~
Teleport to Foggy Forest Path northeast waypoint and head south to the Rusty Rudders. Talk to the waitress near the cannon on top of the ship. The answers to each question are as below.
Question 1: How many tables are there in The Rusty Rudder?
Question 2: How many employees do we have in The Rusty Rudder?
Question 3: What is not served in The Rusty Rudder?
Answer: Romaritime Flower
Once all questions are answered, go to the cannons and fire it once (no need to aim since it’s already in the correct position). An opening to a cave will appear and you can enter it; there’s a waypoint in there so don’t forget to activate it.