Air Moorea 1121 crashed from no more than 400 feet, yet people have survived far more awful disasters, such as UA232 or JAL123.
I'm guessing you consider your examples to be crashes from "90x higher" because the initiating events happened at high altitude. This extra altitude was actually a good thing in these cases, because while the pilots lost most control of the aircraft, they were still left with partial control. The extra altitude gave them plenty of room to figure out what was wrong with the airplane and how to control it with what was still working. This allowed them to end up in more of a crash landing rather than just a crash. In airliners that really do crash from higher altitudes, there are no survivors.
United 232 was more of a crash landing. Although the hydraulic systems were lost, the plane was still trimmed for level flight and the pilots could use the remaining engines to control the plane. They had plenty of altitude and airspeed at the time of the engine failure to get the plane under control and figure this out. This resulted in the crash landing where about 2/3 of the people on board survived.
Japan Air 123 was similar in that the aircraft lost hydraulics but they also lost most of the vertical stabilizer. Again, this happened at a higher altitude which gave the pilots room to react to the issue. The pilots still managed to fly the plane for a while but ended up crashing into the mountains. The survival rate was less than 1% though this was partially due to delay in the rescue operation.
Air Moorea Flight 1121 lost its elevator control cable. While the pilot could have used trim to control the aircraft's pitch, the airplane initially entered a dive and 400 feet was much too low to recover from this. If the cable had broken at a much higher altitude it's possible the pilot could have regained control.
Another point to remember is that in a large airliner like the DC-10, the passengers have a lot around them to cushion the impact. In a small plane like the DHC-6 this isn't the case. A crash into the water also results in a much higher impact, and any survivors would have to be able to swim free to the surface.
From 400 feet or 40000 feet the most important factors in surviving a crash are how fast you are going and how sudden the stop is. In Morea 1121 the elevator cable snapped and the airplane became uncontrollable, pitching down and impacting terrain at high speed, the airplane came to a very sudden stop. In UA232 the airplane was still controllable to a degree, the pilots were able to get the airplane close to the ground and when it crashed the energy was dissipated over a longer period of time. The fuselage of UA232 was able to absorb some of the energy and protect some of the people on board.
Air Moorea 1121 was unsurvivable due to the high G forces of the impact with the water. The aircraft nosed over and dived into the ocean at full power. The loss of pitch control was caused by a snapped elevator cable. If the cable had snapped at a higher altitude, the pilot may have had time to use the elevator trim tab to regain partial pitch control and not crash with such severity.
Those other accidents you mention were very different. In both cases the loss of control started at high altitude and the pilots had enough time to establish partial or limited control. They were able to maintain some control up until the time of impact. The impact G forces were therefore far less and were survivable for many of the passengers.
There's a simple physics principle you need to keep in mind
Increase the distance, decrease the force
A good way to think about high-velocity impacts is not in terms of things (like water) acting more solid, but in terms of things (like people, rocks, Fabergé eggs) acting more fluid. The more energy that’s involved in a collision, the less important the binding energy (the energy required to pull a thing apart) is. A general, hand-wavy rule of thumb is: if the random kinetic energy of a piece of material is greater than the binding energy, then the material will behave like a fluid. A bit more energy, and it will fly apart.
So, when you fall from a great height and land in water there’s a bunch of kinetic energy going every which way. The water continues to behave like water, but since the kinetic energy in different parts of your body are greater than the binding energy keeping them connected, then the body as a whole will act more like a fluid. That is; it’ll “splash” (in the grossest sense).
In other words, the impact victims took the brunt of the impact energy themselves, thus destroying their bodies (i.e. they died). Had the pilot leveled off before hitting the water, the water would have absorbed far more of the aircraft's energy.
This is why, for instance, United 1549 had no fatalities. In this recreation (relevant portion starts about 3:00) you can see the aircraft enter the water parallel to the water and slow down.