If the players head is partially above and below water, all terrain under sea level is visible, along with the terrain above sea level. The effect will activate and deactivate instantly, and has no "warm up" like the new underwater viewing mechanic. The screen shot is from creative mode, with my characters head half above and below the sea.
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After further play testing, it is the same as MC-126113, however the causation between this bug and MC-126113 is different. MC-126113 mentions waiting for the underwater night vision effect to "charge-up" so to speak, granting night vision after exiting the water. What I am reporting is that if the player is half in water and half in air, the night vision effect applies, in full, instantly. And after moving fully into the water or into the air it completely goes away. I do believe what I reported is slightly different however probably caused by the same thing.
I can't recreate the instant full night vision effect unless it was already full and I exit and reenter the water very quickly. When you exit the water, visually the night vision is completely removed, but in the background it is rapidly declining. This means that reentering the water quick enough will make it appear to instantly give full night vision. As for what happens when you fully submerge, the water overlay is applied and makes the vision foggier, meaning things in the distance will disappear. The fix to MC-126113 should remove that issue as the night vision shouldn't apply while the player's eyes are out of the water, unless a night vision potion is used. Does any of that seem to match the issue you are experiencing?
Duplicate of MC-126113 if I understand the situation correctly.