Hi Rafael:
I believe the answer depends on some unstated assumptions. The following are some examples.
What do you mean by "fall off a mountain"? Is your fall off a cliff through the air, or are you sliding down a steep slope of the mountain? How high is the mountain, or more relevant, how far vertically do you fall? Can you control your orientation so that you fall with your body horizontal until you are near the water/snow (to minimize your velocity when you hit the water/snow), and then reorient your body so you enter the water/snow feet first with slightly bent knees (to minimize injuries)? What are you wearing? Assuming the fall itself does not cause serious injuries, you will likely end up deep in the water/snow, so what will you do to avoid drowning/suffocating in the water/snow? Perhaps you have a breathing device and an air supply with you. In that case, if you are in water, then perhaps you can swim to the surface and survive. If you are in snow, how will you get to the surface before your air supply is exhausted or you freeze to death?
Good luck with your story.
Regards,
Buzz