Vanilla Secret 3
From the Super Mario Wiki
Vanilla Secret 3 is a level in Super Mario World. It is located in Vanilla Dome, and it can be accessed through the completion of the level Vanilla Secret 2. To clear the level, the player has to jump on dolphins or swim through the water. The dolphins do not hurt Mario or Luigi in any way, but they can be eaten by Yoshi. Some dolphins jump and dive, while others go vertical.
When the player reaches the first vertically-jumping dolphin halfway across the level, the player will find a Porcupuffer, which is the only enemy in the entire level, and quite a hindrance if the player chooses to swim. Near the end, the player will find coins in the shape of arrows pointing to the right, one of which is a Dragon Coin. The level ends with a green horizontal Warp Pipe, leading to the goal. By completing this level, the player gains access to Vanilla Fortress.