The first star of this course has two ways to go about getting it, the roundabout way and a super easy way. For the roundabout way, jump into the painting at its lowest point, then activate the Crystal Tap behind you at the beginning to raise the water level some. From there, you can swim over to the taller platform with the Amp, where you will find another Crystal Tap. With the water level up more, you can swim over to the wooden platform that’s being held by thin wooden pillars. You will be able to jump out of the water and onto the wooden platform, although it may take a few tries.
There will be a purple switch on this platform, which creates a path to the nearby taller platform, where another Crystal Tap is located. Some wooden platforms will rise with the water nearby, so get on those and you should be able to double jump to the platforms above those. On the second platform, you should notice some small lifts that have directional arrows on them. Those arrows signify the directions the lifts will move. Use those lifts to reach the ledge with the yellow box, which contains the star. Just be careful of the Keronpa Ball by the box, as you don’t want to get burnt and run off the ledge.
The other way to get this star will not have you worrying about the water level at all, except that it needs to at its lowest point in order to work. You know that one series of ledges near the beginning that has an Amp circling it and a Crystal Tap on top? At the base, in the little corner, stand there and you will be warped to the top of the course, right next to the cannon. This is right above the star, so you only have to drop down and break the box to grab it.