It's impossible to save yourself with a water bucket from certain heights, since you die before hitting the ground. You can still save yourself when falling from build limit to bedrock, but somehow not when falling other lower heights.

Steps to Reproduce:

1. give yourself a water bucket
2. type "/tick rate 1"
3. type "/tp @s ~ ~126 ~"
4. spam click as fast as you can, or use auto clicker or whatever
5. die of fall damage