Pokémon Ruby and Sapphire/The Shoal Cave

The Shoal Cave is located just outside of Mossdeep City.

Depending on the time of day there is high tide and low tide. You can confirm the schedule by speaking with Sailor Ernest from Route 125.

Shell Bell
If you collect 4 Shoal Shells from high tide and 4 Shoal Salts from low tide, a man will give you a Shell bell for those, it will restore your HP by 1/8 of the damage you dealt to the opponent.