When you dig straight down about 3 or more blocks underwater it creates an air pocket but you still float down as though you are underwater even though you can breathe

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as this seems to be incomplete but i was easily able to reproduce ill add the way to reproduce here.

create a new world in 18w21a

find a body of water

underwater dig a hole straight down for about 4 blocks should be fine

from above it looks like the hole is full of water particles and water color and all.

but if you go into the hole it will look as if you are outside of the water but you will still see particles.

and you float like you would in water but you don't need to breath

and there wont be an error showing in the logs