We made these a while ago, but both of my boys still enjoy playing with them in the bathtub. These shapes were cut out of craft foam (found at the dollar store). The foam is quite thin, so easy to cut with regular scissors.
It is also really fun to poke holes in, which is mostly what my 3 year old did while I was cutting:
After I made a few basic shapes, my son wanted to do some. This is my favorite, a guy that he drew:
One warning if you try to make this, the red and pink leave color behind on the wall, even after several times soaking them in water. A magic eraser does a pretty good job of getting it off.
I got the idea for this from MomAdvice.