You will be headed to pretty much the top of the course in the first area for this star. You can bypass a lot of platforming by jumping into the painting as high as you can, which will set the water level to its highest position (a Side Somersault works fine for this). Once you’ve done this, swim over to the main structure in the first area, either via the slope or the floating wooden platforms. Jump on over to the area with the one enemy zipping back and forth, which is called a Chuckya. This enemy is similar to the King Bob-Omb in Bob-Omb Battlefield, as it will attempt to grab you and toss you into the drink.
You can either ignore it or grab it from behind, then tossing the creature to defeat it. Anyway, there will be a long plank of wood in the vicinity, which extends to the platforms in the center of the map. The plank isn’t very large, so you will want to be careful traversing it, plus there are two Amps buzzing around near the end. The platform the plank is connected to has a Keronpa Ball, which is designed to light Mario’s butt and make you run off of it, so quickly outmaneuver the flame and jump onto the spinning platform. From there, it’s one more simple jump to get to the yellow box with the star.