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.
Step 3: Make the "whirl" on the pool -> foamy water
Step 4: Add the feet
Make sure to prepare enough "skin", you will also need some for the heads and the arm of the guys later on.
Shape four feet out of the mixed FIMO. I made to small and to large feet, to represent the size difference of our feet in real life :). Then postition the feet onto the water on one side of the box. Once you're happy with the postition, you can smoothen the transition between the feet and the water. But you might want to wait with that step until the heads are positioned too.
Step 5: Add the rest and bake it!
After that I also made a hugging arm from my boyfriend around me. I think it looks quite cute. Here you could also add a tatoo or something to further identify your little guys.
Then put it in the oven and bake it according to the instructions on the packaging of the polymer clay. After baking my whirlpool looked exactly like before.
Put money into the box and use it!
And yeah: the polymer clay seems to stick on the metal box.... It just works!! I was actually surprized too, since I never saw anything similar before, so it was just an experiment.