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.

Items
(Low Tide)
 * Shoal Salt - second cavern in the southwest
 * Shoal Salt - second cavern under the bridge in the north
 * Shoal Salt - middle of the third cavern
 * Shoal Salt - southeast corner of the cavern with stairs/ladders
 * Ice Heal - southeast corner of the stairs/ladder room
 * Focus Band - receive from the Black Belt trainer
 * Never-Melt Ice - central plateau of the Ice Cavern
 * TM07 (Hail) - ledge in the Ice Cavern

(High Tide)
 * Shoal Shell - back cavern, center ledge
 * Shoal Shell - back cavern, northwest ledge
 * Shoal Shell - back cavern, northeast ledge
 * Shoal Shell - back cavern, southeast ledge
 * Rare Candy - back cavern, east side
 * Big Pearl - northeast ledge of front cavern

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.