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
Related issues
duplicates
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Duplicate of MCPE-53361