This is your first underwater level. Mario's movement changes dramatically; pressing A will make him swim upwards, and releasing it will make him sink. You'll encounter Cheep-Cheeps (Fish) and Bloopers (Squids). There are also currents that can push Mario around. A Super Mushroom is usually available. A hidden Carrot is less useful here as Bunny Mario's float doesn't work underwater, but a Superball Flower can be found in a block, which is excellent for dealing with underwater enemies from a distance. Be mindful of your air supply; while there's no explicit air meter, staying underwater for too long without surfacing can be dangerous in some sections.
Swimming Control: Practice short taps of the A button for precise vertical movement. Avoid getting caught in strong currents if possible, or use them to your advantage to reach hidden areas.