This is a very simple boat your kid (or even you) can play with in a pool or a bathtub.
The original model needed you to tape a rubber band to the shaft but I found that it didn't work well. I added a small tab which made it much easier to attach the band. You can then wind it up, put the boat on the water and release it.
xyzprinting da Vinci Jr 1.0
I found that printing the axis at 92% the original size made for a better fit. I didn't adjust the STL for my remix as I'm sure that varies depending on printer and filament type.