The hard part about a toddler like that is that everything needs to be 40x40x40mm or larger so they don't choke. But to your question about a boat...I believe PLA is naturally very buoyant, so a boat should be feasible. You might need to add thinner walls to the print with a little bit of infill for stability, but if the print is modeled right, it should be water tight...

I would start with a wide, flat bottomed boat. Something that would be easy to model and print. Actually, check Thingiverse. I bet someone's already done it.