You could make your own, but that would be disgusting. Instead we propose getting these little wind up Floaters. Good fun and fairly clean bath time laughs can be had when you wind these little turd shaped beauties up and let them go.
If you're not into Racing Floaters then try fishing for them instead! (search for our Fishing Floaters).
In a pack you get two smiley Floaters race ready, just wind them up and let em go!