What I expected: Half-slabs are solid blocks therefore you can't be affected by water inside it
What occurred: When flying into (from under) the bottom half-slab(waterlogged) it produces a sort of blindness effect.
Other issues related: 

• Also possible in survival
• Also works for some other water-loggable blocks
• A night vision potion has no effect on the visual.
  Signs are to prevent water spillage