You have to go into the Shoal Cave twice, once at high tide and once at low tide. Low tide will have no water, allowing you to get to the items on the lower parts. High tide will flood the cave, meaning you will need Surf to navigate.
High Tide is between 09:00-15:00 and 21:00-03:00.
Low Tide is between 03:00-09:00 and 15:00-21:00.