How deep does water have to be to prevent fall damage?

I have a very deep vertical mine shaft (I started in a mountain and hit bedrock), so it takes a long time to get up or down using my ladder. To speed up the trip down I thought "why don't I add a little pool for me to jump in?", but when testing it out the first time I died and had to rush back to my mine to collect my items before they disappeared.

How deep do I have to make the pool in order to prevent myself from taking damage when I hit the water?

Solution 1:

As of version 1.4.3, only a single block is enough to prevent all fall damage.

For previous versions it was 3 blocks.

Solution 2:

While yx_'s answer that the water must be 3 deep to prevent damage is mostly correct, it doesn't actually have to be 3 deep but only three blocks away from the surface below.

Water  
Water  
Water  
[Solid]

will prevent damage, but so will

Water  
Sign  
Air  
[Solid]

This also works with ladders instead of signs, and probably doors too, but ladders can still kill you if you hit the edge of them. Another thing to remember is that the water block effectively resets your distance fallen, so if you fall too far after leaving the water you will take damage.

Solution 3:

After the release of 1.5 I tested this by jumping into pools of water 3x3x1, 3x3x2 and 3x3x3 from a height of 64 blocks. I did this by flying up and then using the /gamemode command to set myself on survival. To my surprise, landing in the pool 1 block deep didn't kill me and I walked away without any injuries. This may be by design in 1.5 or it may be changed in later versions.

Before 1.5 it was 3 blocks to survive.

Solution 4:

You could put one only one layer of water and when you are falling, press and hold space so you will end up swimming. I never really tried it down to bedrock, but it prevented fall damage from like a 64 block tall cliff which I happen to be experimenting a couple of weeks ago that I almost forgot. =)

Solution 5:

You will not take fall damage even if you land in minimal flowing water. I mean that you don't take fall damage after landing on water,no matter the depth.