Sad fact: Benchy doesn't float, but I have 10 of them and my daughter wanted to play with one in a bath. So I quickly designed one very simple floating boat. It floats great and it can carry smaller toys too, including a Benchy :-)

In spiral vase mode it can be printed in 30 minutes (but it is resizable too). 

There are instructions for PrusaSlicer and Cura in images, but point is that you have to enable the Spiral Vase mode and if you are using 0.4mm nozzle, set the wall width to 0.5mm. 

Have fun with your kid :-)