Minecraft 13w42a, Minecraft 13w42b, Minecraft 13w43a, Minecraft 1.7, Minecraft 1.7.1, Minecraft 1.7.2
If you stand in a 2 deep (or more) body of water, it is quite bright underwater (without Night Vision or Respiration enchant). There is also no distinguishable border between land/water.
It might be a new feature, but it could also be a bug.
Sorry if this issue has already been posted: I find it hard to find previous posts about an issue.