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.

During high tide, you can catch and, rarely. During low tide, the Pokémon are more powerful as you go deeper into the cave. When you get to an icy area, there is a rare Pokémon called which will evolve into  later on.

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.