From the Super Mario Wiki
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Mario starts off near the door outside of water but there's nothing he can do outside the water so he must jump in. In the water Mario must swim around while avoiding Bukubukus which once in their sight charge at him. Mario can swim to the left for a 1 UP Heart but the main stuff to do is at the right. Mario must swim right while avoiding a Bukubuku then swim up to the road to activate it then connect it to the platform with the key so he can jump onto it. Mario must get onto a single platform quickly grab the key and run to the door before the road ends. Once at the door with the key, the level is completed.
Pauline's lost items
Within this stage, Pauline's lost items can be found in the following order and in the following places: