How do you prevent death if your parachute does not open?

You land in the water! (How does one LAND in WATER?) Aside from the semantic issues, impacting the water at 120 mph (your terminal velocity in a spread-eagle posture) is as likely to result in death as hitting soft soil or hard rock. The course of action that you need to follow is to avoid hitting anything at 120 mph. That is accomplished by deploying your emergency or reserve chute. Before you deploy the reserve chute you must cut away the failed chute so as not to get them entangled. Safety begins on the ground where you pre-flight all of your gear and receive appropriate training. If you are planning to hit the water, be sure to, right before you hit, straighten your body and enter feet first with your legs crossed. That will keep your legs from being folded up around your ears.