The problem was never the water but the landscape’s LODs changing to be far less accurate at a diatance.
If you use the landacape/water system, this is what you get with shallows.
You can try to brute force the LODs from shifting up/down too much by setting the height of the shallow at exact mathemarical even numbers.
So the Z would be -10 instead of -10.53610 for instance.
Still, the LOD will drastically mutate, causing effects like this when intersecting planes.
A quick test to verify the issue is to place a plane with a black material in place (at the height of) of the water.
Not much else you can do short of not using landscapes.
Particularly in your use case, a C like shape is always going to simplify out to be filled, so you just need to make your landacape different…