Further to this. It appears on my world map, the water isn't rendered at all. Like basically, you can see through the world to the sky. It's definitely strange.
I tried disabling XLODgen, Dyndolod, Texgen and no change.
https://imgur.com/FHnS3fD
If you look here in particular, all you can see is a Skybox. No water actually rendered. So something somewhere at a distant setting just isn't putting water in.