The top of these blocks are clearly above the layer of the water, so I'm not sure why it unlocks - being here for example isn't sufficient to activate riptide or other water exclusive effects.
May be a tad difficult to reproduce if you've already unlocked this advancement in a world.
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discovered while testing 1
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This seems to happen because it is caulculated as if water would be a full block even though it is 2 pixels shorter, simelar to MC-185955
Relates to MC-1448.
muzikbike
Can confirm for 1.16.5