To put it simply, water gets pushed out of the way by your body when you jump in, but it can only get out of the way so fast. A fall at thirty miles per hour into water from a 10 meter diving board is already risky. Meanwhile, the average human body falls at roughly 85 miles per hour from 220 feet, the height of the Golden Gate Bridge. Surviving from heights higher than that is nearly unheard of; when you reach 120 mph (terminal velocity of the human body) it's pretty much game over.
In real life, a person who falls into deep water and enters it in a streamlined position will fare better than hitting ground because they are making a gradual rather than a sudden stop. However, there will still be a lot of force exerted on the person's body, given that water is about 800 times as dense as air. So if you've ever been hit by 120 mph (55 m/s) wind, just imagine if the air was 800 times as thick to get an idea of hitting water at terminal velocity. To make matters worse, entering a feet-first diving posture (even professional divers don't generally do head first diving above 65 feet) in preparation for entering the water will increase a person's terminal velocity, making impact energy even higher. However, with this technique it is possible for even a minimally trained person to survive jumps into water from around 55-65 feet (Navy personnel were trained to abandon ship in this way).
There's also the issue of one's swimming ability — many people who are fortunate enough to survive the fall itself will often drown because of poor swimming ability or injuries will make them unable to keep their head above the water (particularly if knocked unconscious upon impact). Keep in mind the deeper you are underwater when you stop going down the farther it is back up to the surface — and hitting the water likely made you exhale. Then there's the issue of exhaustion, as even the fittest and most skilled swimmers simply cannot tread water forever. Or, if the water is sufficiently cold, you're likely to succumb to hypothermia and drown before help can reach you.
In short, a fall into the drink from 200 feet or more will very likely be fatal — whereas a fall even onto "solid" ground may not!
In some cases Hollywood water can be justified because in particular places water really does become softer. Kayakkers have paddled off waterfalls over 180 foot high and survived because the water coming off the falls breaks the surface of the river below and introduces a lot of air into it, which comes up as bubbles. They're not actually landing in water, but in a water/air mixture with a lower density than just water. This of course does introduce a completely new set of problems because it means the falling person will end up diving to a greater depth than they would have done in normal water and thereby increasing their chances of hitting the riverbed, especially when not using a kayak or another device with several hundreds of liters of air inside to help one avoid such a fate (such as the barrels used for going down the Niagara falls).
This goes in both directions. A character or a building hit by a wave crashing over them often won't get as much damage as they would in real life, despite the fact that one cubic yard of water weighs nearly a ton. (A cubic meter of water weighs a tonne — a metric ton — equal to 2,204.6 lbs.)
Note: "Soft water" also won't stop bullets, which visibly penetrate water quite deep with prominent "bullet-streaks" and fatal strikes. However, MythBusters demonstrated that no supersonic bullet can penetrate more than 3 feet (91.44 cm) of water; their sheer speed causes them to shatter once they enter the water. Subsonic pistol rounds do better, penetrating up to 8 feet. And any other bullet's trajectory will be thrown off in a large body of water. A target is unlikely to be hit anyway, since light-diffraction from the water makes their location appear slightly off, disrupting a gunner's aim and the impact of the bullet at the water's surface will likely change the bullet's trajectory to some degree.
MythBusters also demonstrated that even at very high velocities, water is significantly softer than concrete (it's the difference between breaking all your bones and breaking all your bones and being decapitated from the impact) — falls that would be fatal on land become survivable in water. On the other hand, you might survive the impact only to drown moments later if your injuries are so great that you lose consciousness or otherwise can't swim to safety, as mentioned above. And while water will always be softer than concrete, "softer" is a relative term—the shock of impact may still be instantly fatal if the water is entered at a high enough speed.