They showed the worse use case. The current water is meant to be contained, not dumped on flat ground. Neither RWG or POIs have water dumped on flat ground.
The water mesh generator is the old mesh code with some cleanup and the programmer working on it is still making changes. The mesh generator makes meshes at a fixed height, which does not tapper off currently.
1 - A20.7 has a few minor changes to reading the world file header. It will not effect existing A20 saves. It will simply show A21 saves in the list without throwing errors.
2 - A20.7 saves are the same as 20.6 saves. A21 will not read those saves due to many changes that are not compatible.