You are absolutely right, I shouldn't have put such a hard obstacle so early in the game, it seemed obvious for me because I spent hours experimenting with the mechanics before making the level, but I agree it isn't for players playing it for the first time. I think I will redesign the whole tutorial, to show the color change mechanic earlier, at a stage where your whole snake fits in the screen.
I kept adding more effects to the water all along the jam to make it stand out more, but yeah it's still too dark to be clearly visible, I'll keep working on that !
Thank you for the feedback, it will help me make the best game I can !