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CONTAINS SPOILERS
Secrets at the docks nearby The Stranded Sailor
Jefferson provides the clue "If you have a way to get across the water, there is more to explore along the docks nearby." He's referring to the Hook Shot spell. Walk on the piers to the east and west. Use the Hook Shot to get across to discover the secrets. North of the docks is a cave with an entrance that is not obvious. Just keep walking until you enter the cave to get the secret.