Use the raft to reach the island and then use your rope on a rock to reach the waterfall.
Since "The Waterfall" can refer to several popular topics, here are the most common guides to help you: The Waterfall
Talk to near the Baxtorian Falls (north of Baxtorian Outpost). Use the raft to reach the island and
Enter the tomb (bring no weapons or armor!) to get . The Waterfall
The is a famous members-only quest in Old School RuneScape (OSRS) known for giving massive experience in Attack and Strength, allowing level 3 players to jump straight to level 30 in those skills.
Return to the waterfall with the amulet and runes to complete the quest at the Chalice of Eternity. [Quick Guide] Waterfall quest