Pokémon Ruby and Sapphire/The Shoal Cave

The shoal cave is outside of mosedeep.

there is a high tide and low tide

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 items 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.