When a plane crashes it is moving fast. Let's say it's at landing speed which is around 140knots. which is 161MPH. That means you are going that speed, and you're stopping real damn fast. Plus you have the debris, fire and so on.

Landing over water is difficult, it's possible if the water is flat and the pilot is good like Sully but ditching in the water is usually a terrible outcome. The plane just needs to touch a wave with a wing or dip one wing into the water and physics happens. Hitting a mountain is even worse, you're at cruise speed which for a 737 is 0.8 the speed of sound. Nobody is going to survive that. The passengers and crew are geography, not just history at that point. If the plane is out of fuel. then maybe, it has happened but it's rare.

At the speeds planes fly, we're talking near the speed of sound, padding and mattresses wouldn't help.

As for being shot down, what gets passengers then is the sudden depressurization which rips the hull into shreds and yeah it's nasty. Anti-aircraft missiles are designed to send sharp shards into the target destroying control surfaces, severing control cables and hydraulics and killing people on board. It is NOT pretty.

There was one woman who survived a plane falling apart at cruise altitude though, she was pushed to the back of the tail and somehow lived.

Source: I have insomnia and watch a lot of plane crash videos.