pyxelizeBeater: The reason why the skybox breaks is because snakewater_final1 has been edited throughout the years instead of changing name, so when you load an old demo that uses an older version of the map and for some reason that breaks the skybox
you can fix this by replaxing your snakewater_final1 file in your maps and download folder to the correct version for that demo, I think snakewater_u18 is usually correct
Thanks, cp_snakewater_u13 fixed all my demos, also thanks to Beater who kindly provided me with this version of the map since it seems to not exist anywhere anymore.