Let's separate the terns you've used in your question and comments.
This is a very ambiguous phrase and doesn't really describe a flight phase or operation whereas auto-rotation and water landing do. The pilots operational manual will simply say something like "land as soon as possible".
An emergency descent means that you have a problem which requires you to get on the ground as fast as possible, not including all engine failure since that is an auto-rotation.
The fastest way of getting any helicopter down is to enter auto-rotation and if I really had to get down fast, that's what I would do. In the last few feet though, I'm going to get the engine(s) driving the rotor again so I can make a normal landing.
A true auto-rotation does not have an option of a powered landing, since by definition you have no engine(s). If you have power, then it's training, practice, testing or precautionary. Everything you do until the last fifty feet or so is making sure that you manage the rate of descent and airspeed to keep the rotor in the operating range. The end goal is to arrive at fifty foot with enough airspeed to be able to flare so that you can trade your kinetic energy from airspeed to kinetic energy in rotor rotation so that you can increase pitch and therefore lift and therefore drag in the flare, whilst feeding in kinetic energy to stop the drag from stalling the rotor.
It's all about energy conservation and conversion. You start off with some potential energy (height and mass) and some kinetic energy (airspeed and rotor RPM). An auto-rotation instantly starts converting potential energy into kinetic energy - you start going down.
Take two identical helicopters flying in identical conditions except one is at minimum weight and one at maximum weight. The one at maximum weight has more potential energy since:
Where M is mass, G is the acceleration due to gravity and H is height.
Both helicopters enter auto-rotation. Quite soon, both will stop accelerating downwards and become stabilised with a constant rate of descent. The heavier helicopter though needs more lift to support it's weight. The airflow in auto-rotation always comes up through the disc from below. You can gain the extra lift by allowing the rotor RPM to increase or you can increase the angle of attack of the blades by raising the collective. The rotor RPM always have an upper fixed limit so in practice, the only way to gain the lift, and support the extra weight, is to raise the collective to prevent the rotor from over-speeding. You also have a limit to how much pitch you can use as the angle of attack must remain positive to the relative airflow from below. If you try to reduce rate of descent by continuing to raise the collective, you will lose lift as the angle of attack moves towards the critical angle so eventually, you reach a steady rate of descent, greater than that of the lighter helicopter.
Since the heavier helicopter is now generating more lift, it is also now generating more drag which can only be overcome by converting more potential energy into kinetic energy. Therefore, the heavier helicopter will use up it's potential energy faster and, since the rotor cannot be 100% efficient, this means that the rate of descent will be higher than in the lighter helicopter.
(It's a very complex topic beyond either the scope of this answer or, to be honest, my complete understanding but some helicopters, in some weight ranges in some flight conditions can descend more slowly when heavier due to rotor efficiency and the size and location of the "auto-rotative driven region" vs the stalled region of the disc).
So, when you arrive at the bottom of the auto and it's time to flare, you have a high rate of descent in a heavy helicopter and the only way to arrest this is to increase collective pitch, to increase lift and increase drag. The only power available to you is in your kinetic energy so you must trade airspeed for rotor RPM in the flare. So a higher rate of descent means you need a higher airspeed to make a controlled power-off landing.
All of that explanation was only to explain that a heavy helicopter arrives at the bottom of an auto-rotation with a high rate of descent and high airspeed.
I couldn't find a video of a Sea King auto-rotating, probably because heavy helicopter pilots don't practice auto-rotations like private and sports pilots do. But here is a heavy helicopter coming in to land. Notice how fast it's going when the flare starts, how high it is and for how the long the flare is held. Even after that, it still lands with a significant rate of descent and airspeed.
S-92 helicopter autorotation (power off landing)
In a light helicopter, especially with some wind on the nose, a good pilot can land with zero airspeed and just enough rate of descent to gently touch down. You could not do that in a heavy without a lot of wind.
Now for the unclear bit.
If you mean landing on water at the end of an auto-rotation from normal flight, then clearly, a heavy is not going to fare well with that arrival so unless you have no choice, you're going to go for firm ground.
If you mean a hovering auto-rotation, assuming that you are within the limits of the height-velocity curve, then you have no choice and it's not going to end well unless you have flotation devices which are deployed and you make a really good touchdown.
If you mean the "water bird", then this is neither an emergency descent or an auto-rotation but is a single engine landing, therefore powered, where you use the boat hull shape of the fuselage to arrive at a precise speed and pitch angle to allow the hull to plane and cushion the landing to enable the helicopter to settle into a float.
What a long way of saying that emergency descents, auto-rotations and water bird landings are very different things and therefore not analogous.