When Swimming in underwater ruins, sometimes you will be able to breathe. the bubbles end up going away. this only seems to work in some underwater ruins and on the ground.
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Pretty sure this is a dupe of MC-129358
In MC-129358 it mentions that you can see the outline of block selection, unlike normal water and if you brighten up your screenshot you can see the outline of the moss stone brick.
Please provide the seed and the coordinates.