Charge Stone Cave is a specific location inside the Undella Bay area, not a separate cave. It's accessible from the Undella Bay beach by following a path down to the right.
If you are trying to get somewhere else from Undella Bay, you might need to:
* Use the exit leading to the beach: This will take you back to the main area of Undella Bay.
* Use the exit leading to the city: This will take you to the main city area of Undella Bay.
* Use the exit leading to Route 16: This will take you to Route 16.
Let me know if you have any other questions about getting around in Undella Bay or other areas in Pokemon Black!