First, I placed myself in an ocean and made sure the the chunks at the end of the render distance were ocean as well.

Then, a short time after /time set 12900, the global lighting does a sudden shift and everything suddenly gets darker.

This is likely being caused by the sun's lighting source dipping below water level at the edge of the rendered chunks. When it passes behind blocks in the horizon, it manages to be smooth yet in oceans this happens instead.