I’ve noticed this problem a few times. The Create Water device does not seem to be as accurate as the existing water from my Coastal Erosion (comparing to the mask). The screen shot shows the water mask overlaid on the terrain in Scene View with water.
Okay, I took a hint from my second screenshot and looked at the Create Water device output, directly. Here, I can see the correct ocean inlets. Unfortunately, there is still a problem with the rivers coming into it (and with the mesh integration between land and water.)
Well, yay for me! This turned out to be one of those times where I figured out the solution on my own. Turns out I needed another Coast Erosion between my ocean floor erosion and my land erosion. Plugged into the Flow Restructure and Create Water devices, it all finally matched up.