So if you will use Coast Erosion set to 619 m height
And Height Selector top point of which set to 619 m (no fallof also)
And will overlay it over scene preview node (in may case HS used for that sand line Bitmap creation) - you will see that Coast Erosion water is lower, than Height Selector
It is not good - because i need to see actual water while work. I did think that sand over water is bug. But no - it looks like bug in Coast Erosion
If you want to make them on same level, you need to still have Height Selector set to 619, and change Coast Erosion set to 619.999 - only after that you will see correct water level
Also sorry if issue is my knowladges, maybe i really did a mistake somewhere