Well, if my sense of direction is right, this is the same area where that yacht from the marina vanished.
6 seconds sound clip from Scooby-Doo, Where Are You! - A Clue for Scooby Doo (S1E2).
You can hear this line at 00:16:59.853 in the Blu-ray version of the episode.
Quote context
[...]
- I found a clue! I found a clue!
- It's a secret storehouse of scuba tanks.
- Things are beginning to add up.
- You mean like two and two are five?
- I'd like to get a close look at that ghost.
- But his trail's not. Look. Glowing footprints. Now all we got to do is follow them.
- The footprints vanish right into the side of the cliff.
- You're right! Shaggy, you're a genius!
- I am?
- Sure. Who else but you could have sat down on the rock that opened the secret passage?
[...]