The bug
If you place bone meal on a floating water block, sea grass is spawned with a full water block on this position.
MC-127996
Bone meal can be put on floating water
Resolved
Fixed
5
Confirmed
bone_meal, sea_pickle, water
Minecraft 18w14b, Minecraft 18w15a, Minecraft 18w16a, Minecraft 18w19a, Minecraft 18w19b, Minecraft 18w20a, Minecraft 18w20b, Minecraft 18w20c
Minecraft 18w21a
Retrieved
The same works for sea pickles and coral.