Thank you so much for the detailed feedback! I totally agree on all points.
Your partner is right, the fishing is on an (I believe) five-second timer that's tied to a state. So you need to begin the fishing animation shortly before the fish state starts, which is when the fish appears in the pond. It's not very intuitive, I plan to improve it in the future, but it's what I could figure out for the time being.
The background is what I set Godot's empty space color to, which was before I made the cave. You're absolutely right, and it bothers me too. I'm not sure how to change it on a per-scene basis. If I can't, I can probably create a workaround by either extending the cave scene to fill the camera space, or adding a color rect or something that exists beneath everything else.
Again, I'm so happy you enjoyed it! It's inspiring to see such positive feedback on what is barely a game so far, but that I've put a lot of work into.