Catamaran

This is a very simple toy catamaran - but you can add your own components to it. Print two pontoons and two bridge pieces. You'll need eight m3 x 10mm bolts to put it together. There are holes on the bridges allowing you to add on your own stuff - solar panel, motor mount, and so on.

I'd recommend printing with at least 1mm walls for water-tightness, but without infill, since you want to keep them light. You won't need supports if you print the pontoons upside-down and the bridge pieces on their flat edges.