When the camera is underwater, certain visual effects are applied—the FOV is reduced, fog renders closer to the camera, and the sky and fog colors are drastically altered. However, these effects are often applied when the camera is inside a water block, but not below where the water's surface is rendered. The altered skybox is very noticeable (and quite jarring) when not seen through the water surface.
A similar problem exists for lava—the extremely close orange fog can sometimes appear before the camera is submerged, or fails to appear until the camera is a short distance below the surface.
This behavior occurs in first-person and third-person camera modes, but it's easiest to observe in third-person mode, as the camera height can be controlled with precision.
Note the attached screenshots. The camera is clearly above the water in both, with only a tiny difference in its position resulting in a very different view..