fins are key to stable vertical flight. The lighter weight the rocket is the better (higher) it will fly. Pressure is required, but don't be near the rocket while it pressurizes. Be sure the trapped bubble of air will drive out every drop of water. Make sure your nozzle is a little smaller than the competition and make sure it needs about 10 lbs more pressure than the competition to launch. Don't expect to hit the right formula on the first try. NASA shot thousands of rockets before they could orbit one. Experiment with several and pick the best.
The formula for fins is the "center of gravity" must be higher than the "center of pressure" of the rocket. To find the center of gravity, find the point on the side of the rocket where the rocket will balance on your finger. To find the center of pressure, cut out a silhouette of your rocket out of corrugated cardboard. Make sure the corrugations go sideways across the cut-out. Use coat hanger wire and a fan. Keep moving the coat hanger wire into corrugation openings closer and closer to the fins. When the cutout begins to spin in the fan stream, that is the center of presssure. Remember the rocket has to coast for a long time with no water in it. so the calculation must be done with the rocket full of water and still be true when the rocket is empty.