The bug

The underwater fog doesn't hide chunk edges correctly, causing unwanted visual effects when the player has a low render distance. If you're below the sea level (most of the cases) you'll be able to see the black background, unlike the surface fog rendering the background with a light color hiding chunk edges seamlessly.

If you're underwater above the sea level, you'll see a blue background instead of the black section, which looks a bit better but you can still see the edges, and this is due to the skybox rendering overlapping the fog causing other issues like being able to see the sun underwater (MC-152504).

Undewater with 16 chunk render distance (fog covers everything):

Underwater with 8, 4, 2 chunk render distance, limits are visible:

Surface to underwater transition at 2 render distance:

Notice how in the first image everything looks fine, then in the second image the fog covers almost everything of the background like it should, then we have the third image where after some seconds the skybox is visible.