I'm getting a strange problem deep underwater in bravil and the ocean near anvil. At a certain depth I get a false "ground" you can swim through it but it obscures the real seabed. I've tried turning off the mods I use "except for the official ones" but it hasn't helped. Has anyone else gotten/fixed this?

It's present in Vanilla, it's due to the actual land being higher than the LOD land. It is uncetain that even updating the LOD land for those cells would change it since the normal land is already so far below the normal surface. Since there isn't anything out there, there isn't much reason to fix the issue, or even be concerned about it. You actually wouldn't see very much of it unless borders were off.

That's somewhat sad to hear but thank you for your help. I'm sure I wouldn't have even noticed it but I recently installed a number of underwater mods. Does anyone know of a safe way to change the depth of the LOD land?

By updating the LODs. But as I already said, that may not accomplish anything since there may be some limit to how far under 0 things can be placed.

Basically this deals with how LOD landscapes work. The LOD meshes are always active, except the portions for the cell you are in are significantly lowered so that the actual landscape can be seen. The height that the LOD land is moved to when you approach it may be limited to a certain depth, Just like how you can only lower land to a certain depth before you have errors.

If you're determined to try fixing it, you will need to regenerate all the LODs for those cells. This is a lengthy process which creates several thousand files (one for each cell). There are a few tutorials around which can explain the exact process.

Thanks for the information.