When a water block is exposed to sky light and that light is then obstructed, without causing a block-update of the water, then the water will continue to have sky light.
This means that if all of the light is blocked from the area, the water block will still “glow” with sky light. This will continue until the water block is updated or until one of the spontaneous updates/light fixes occurs.
Attached are screenshots of a machine for reproducing this bug (built in creative, but it occurs in either mode). In the "open" image, the only source of light is the hole at the top, shining down onto the water. In the “closed” image, I have used a piston to block off the sky, but the water remains glowing with sky light.
Note — This behavior is dependent on the pattern of blocks around the opening to the sky, and removing some of the blocks may hide the bug. In case it matters (I haven't checked), these screenshots are facing EAST (3 / -90°). I originally noticed the problem with a different configuration: a wide layer of water on top of glass with sky light shining on the center of the water, where light would continue to glow from underneath the water.