According to the Tibia Wiki, Waterfall Quest was introduced in the game's 2007 Christmas Update numbered 8.10. The ability to swimming has been introduced in this update. In my opinion, Waterfall Quest is a kind of demonstration how you can use swimming in the game when designing a quest. I think that is why so much effort was put into creating this quest.
Unfortunately, I don't think we'll find any information about this quest in the books and teasers from the update. But... This location connects to another quest introduced a little later in the game. We are talking about the Treasure Hunt quest. In the cave under the waterfall you will find part of the map that you need to complete this quest, If we want to find out more about the hideout under the waterfall we need to talk to NPC Klaus. We need to ask him about Capitan Nathaniel while doing the Tresure Hunt quest.
Player: captain nathaniel
Klaus: Last thing I heared he was in command of a really huge ship. People say it was destroyed in a seafight between Darama and Tiquanda. Maybe you'll find what's left of the ship washed up in one of the smuggler caves in the mountains.
So we know that the location under the waterfall is a smugglers' hideout. In the past, there was a sea battle in the nearby sea. One of the ships was sunk, and Captain Nathaniel, while rescuing himself from the sinking ship, probably managed to reach this hideout and hide his part of the map in the boat. That is probably the whole story of this place.
Sources:
Questy: Treasure Hunt Quest - Tibia ~ Tibiopedia.pl
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