Is the matchbox you used for your paddle steamer waterproof? If not, if it is made of cardboard, for example, you may find that your steamer will sink after a while.

Can you think of a way to make the matchbox waterproof?

Your paddle steamer should work quite well, but it is not very big. Can you design and make a bigger one? You could adapt your design to make a life-sizes duck, for example, that could move across the water.