When looking around in the latest snapshot: 21w10a and the new waving water and leaves, I noticed that the if you have waving water and flowing water next to each other, graphically it shows only the blocks under the water in-between the waving and flowing water blocks.

Steps to reproduce graphical issue:

In a new snapshot world, use https://gist.github.com/felixjones/d5bec1ab0c83ee134fa43a142692a09b#file-blusteryday-zip 
Make sure graphics are set to either fancy or fabulous. 
Find water and cause flowing water next to it. 

What I'd expect if you follow those steps:

I'd expect the water to be seamless without the graphical issue and without seeing the sand below in my example screenshot. 

What actually happens:

It looks like a disconnect between the flowing water block and the waving water source.