Jumping from a height is risky. There are few heights accessible that increase the liklihood of sudden death. Jumping from bridges may result in drowning instead of impact death. I am telling you this because I want you to have everything in consideration. If drowning does not bother you, some people secure heavy weights that cannot be easily dettached, that way the body will be pulled underneath and they will likely be unable to tread water with the injuries (if they survive impact). Also, if anyone sees you, you better believe that boats will be sent immediately to assist.
People have survived jumping from the GGB. Please be aware, this may not result in a quick death.