Plastic milk bottles cut in half along their length are fantastic for making boats. I have made a wooden dowel mast fixed to the bottom of the boat using a piece of blu-tac. (A better way to attach the mast using a block of plastic foam is described below). A card sail has been added as well as a pirate.
It floats really well but can easily topple over so it needs some small stones or pieces of plasticine as ballast to make it more stable in the water.
With a larger bottle you can try more adventurous designs and more masts and sails. You can even model a real sailing ship by copying the position and number of masts and shapes of sails.