This is a repost of MC-126269 for Bedrock Minecraft
The increased brightness effect given by entering water applies a much brighter view of areas that would be much darker in normal circumstances. Simply pouring water anywhere (ie. cave) will function similar to the levels of night vision.
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is duplicated by 7
relates to 2
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This continues to affect the current full release and the 1.17 betas, and makes it so that you can see clearly in absolutely pitch black areas, both underground and at the ocean floor.
I personally like the higher brightness underwater. It really helps make underwater caves look beautiful. I'd say all underwater vision needs is the texture overlay
Affects current Preview, 1.21.50.25
A new observation for this bug is that the time of day seems to affect visibility, even if the player has zero exposure to the sky. To replicate:
1) Create and stand in a player sized box with no light exposure.
2) Fill it with water
3) Toggle time of day between day and night
Inexplicably, it will get brighter and darker within the box.
This bug is still present in preview 1.21.80.25.
As detailed by above, this bug gives an unlisted night-vision effect to the player anytime they are underwater. It is connected to MCPE-84513, which also gives the player night-vision that directly coralates to their vertical height.
Time of day affects the intensity of the night-vision granted, regardless of whether or not the player is underground.
This bug should be considered a parity issue, because water does not at all behave like this in Java Edition.
By the look if it, there doesn't seem to be an attempt to fix this issue. Perhaps it's insignificant or too difficult to fix?