It's actually very simple to make and it looks really nice! It took me about 1h to make it (inclusive baking time of the polymer clay).
Step 1: Material
I took FIMO which is a brand of polymer clay. You can shape it and mix colors like with putty, but after making the shapes you can put it in the oven, bake it and then its hard. The nice thing about it is, that it's not changing size during baking, so usually it won't form cracks or anything like that.
Step 2: Cover the (tin) box
Now you can start to make the surface of the water a bit wavy, so it looks more natural.