Here you have some first sketches of my proposal which consists on having the graphic of the Earth's continents. Everything else would be transparent or with a little bit of blue color. This way, when you fill up the bottle with water, the 'oceans' would be there.
The water you put would look like the water of the oceans. I dont know if it could also work for make users think about how they use the world's water, when they realize the level of water in the bottle is going down.
I'll apreciate your coments. Thnx.