You need: 1 cork, 1 sharp knife, 1 cocktail stick, 1 penny and a leaf or piece of paper.
Just cut the cork in half.
Make a slit down it's curved edge to hold the ballast (penny) and a tiny slit on the flat edge to put the cocktail stick in.
Then find a leaf and stick it on. If the boats keep sinking try a lighter leaf or dry the one you had if it floated originally.
You could also use pretty paper, fabric, ribbon etc but what I like about this is that if you are out camping, chances are you'd have everything to make this (you can use a small stick instead of a cocktail stick) and the kids just love that you can make it from the things all around you that they have found.