Hey there 
Firstly, be sure to read the following post before submitting another bug report. We ask that you only report one issue per thread.
The issues with falling onto blocks near water and not taking damage have already been reported and logged.
There is an issue with players taking damage when falling onto water-logged blocks, such as the sea grass in your video, this has been logged.
As for players not taking fall damage after using a launchpad - this is intended. If you do not land directly on the launchpad, or walk off it before you get launched, you will take damage, this is also intended.