Instructions
Head to Mossdeep City by using the "Surf" TM ability from Lilycove. Go east on Route 124 to reach Mossdeep, then head north on Route 125 to make it to Shoal Cave.
Either wait or set your clock for 3:00 to 9:00 a.m. or p.m. to initiate low tide. Enter the cave at low tide.
Hunt around the sea bed and you will find both Shoal Salt and Shoal Shells. Collect four of each to bring to the old man at the head of the cave. He will give you the hold item, Shell Bell.