The best shape for the nose of a water rocket is a parabolic cone. This may surprise you because most aerodynamically designed rocket noses are shaped as conical cones. This is because at hypersonic speeds, a conical nose is the most efficient design. However, at subsonic speeds, a parabolic cone is more efficient. You can create a parabolic cone by using a technique called "guppying."
Drill a hole through the centre of a large plastic bottle cap using a drill and a 10mm bit. A 2-litre soda bottle will work for a small water rocket.
Fit a car tire valve into the 10mm hole. A standard car tire valve should fit snugly. Attach an air pump with a pressure meter to the tire valve and pump until pressure reaches at least 70 psi.
Put on a pair of safety goggles.
Switch on an electric stove burner and set it to a low setting. Hold the bottle by the tire valve 8 to 12 inches above the burner. Slowly increase the temperature of the burner. As air pressure inside the bottle increases, the bottle will become hemispherical and then will stretch to a more elliptical shape.
Immerse the plastic bottle in water as soon as the bottle has attained the desired parabolic nose shape. If you do not cool the bottle by placing it in cold water, the bottle will continue to expand after you have removed it from the stove.
Cut off the bottom half of the bottle with a hacksaw. Smooth the edges of the bottom half with fine sandpaper. Use this piece as a nose cone. Alternatively, use the entire bottle as a rocket.
If the bottle stretches into an asymmetrical or otherwise inappropriate shape, discard the bottle and try again. You will not be able to reshape the bottle.
Heat the bottle slowly and remove it from the heat before it stretches too far. If you heat the bottle too much or too fast, it could explode. Always wear safety glasses when working around open flames and objects that might explode.