We had a lot of internal debate while designing the tutorial. Our goal was to create a non-intrusive experience that gave players a sense of reward after figuring out the mechanics on their own and solving the puzzles we wanted them to feel good about accomplishing things by themselves. However, we got a lot of feedback saying the tutorial wasn't intuitive at all. Because of that, we added some wall art to offer hints, along with an indicator for the boomerang movement from the second stage onward. Still, we couldn't quite balance the levels and their respective difficulty curve well enough for players to grasp the mechanics in a smoother way...
Either way, we already have several ideas on how to fix these issues and, thanks to your feedback, we were able to outline a solid solution for them :D
Thank you so much for taking the time to review our game! We're glad you liked the overall concept and other aspects of it, and we're really sorry for the frustration.
We actually have a section right below on the game page explaining the mechanics to help anyone who gets stuck, but we failed to point that out, which was an oversight on our end just like pointing out how to switch to full-screen mode to prevent mouse scroll bugs.
In any case, thank you very much for your feedback!
By the way, to beat the first level, you need to make the black box disappear using the Gibbous Light to reveal the door, and then raise the water level using the Full Moon Light enough to block the eye that prevents you from going through the door.