From StrategyWiki, the free strategy guide and walkthrough wiki

The shoal cave is outside of mosedeep.

there is a high tide and low tide

during high tide, you can catch Spheal and Zubat, rarely Golbat. 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 Snorunt which will evolve into Glalie later on.

[edit] Shell Bell

If you collect 4 items from high tide and 4 items 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.