It was discovered during survival map play, the water was blocked by the lichen, so the water was floating right above that like ladders. and when I fell under the water, I fell directly to the floor and got damaged.
After the discovery of this phenomenon, I experimented with lava on a different map, and it was the same result.
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Duplicate of MC-212116, which was later resolved as, "Works As Intended". Please see the parent ticket for further details.