You can see underwater, even if there are no light sources, even if you are locked in a flooded room.

What I expected to happen was...:
I expected the place to be completely dark, since there were no light sources and i had no enchantments or effects on myself.

What actually happened was...:
What happened was i gained visibility with the same light level as if it was day or as if there were light sources nearby.

Steps to Reproduce:
1. I got into creative mode
2. Set time to night
3. Got underwater
4. Surrounded myself with blocks while underwater, still able to see
5. Changed game mode to survival, no changes
6. Changed back to creative
7. Removed water by putting blocks
8. Locked myself inside an empty room
9. It got darker as expected
10. Added water, i gained visibility