When swimming vertically through water the screen flashes and the breath meter flickers. This refills the breath meter.
The effect seems to be that as the player rises from one block to another it's registering temporarily (1 tick) that the player is above water.
Linked issues
Comments 4
I did use the search function but could not find the original post. Keyword phrases used were:
"breath meter refills when swimming"
"swimming underwater refreshes breath bar"
A suggestion in the future - start with the fewest keywords possible. It requires that all of the words appear and given people's different verbiage it can be hard to find 5 words at a time. Also, it will do partial word matches (although partial of all words), thus "swim" will also pick up "swimming".
Duplicate of MC-48395 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.