Instructions
Fly to Mossdeep City and Surf north onto Route 125. Enter the Shoal Cave.
Examine the tide inside the cave. If the tide is high (between 9am and 3pm or 9pm and 3am, in terms of in-game time), Surf northwest and go through the opening. If the tide is low (between 3am and 9am or 3pm and 9pm), head north and go through the lower opening.
Collect four Shoal Shells if the tide is high or four Shoal Salts if the tide is low. Return to the cave when the tide changes and collect four of the Shoal items you did not collect earlier.
Speak to the man who is to the north of the cave entrance. Give him four Shoal Shells and four Shoal Salts. He will present you with the Shell Bell.