Water is supposed to block sky lighting and get it to decay by 3 for each block to eventually 0, meaning it should be dark at the bottom at the ocean. However, this is not how water is currently behaving.
I have included two screenshots showing how I'm clearly able to see the bottom of the ocean at day time when swimming at 1-3 block deep under water like I'm having a night vision effect (screenshot on the left), and, when going deeper, everything slowly becomes dark and turns back to what it's supposed to look like (screenshot on the right).
Nonetheless, it's only a visual bug depends on where the player is underwater and it's less noticeable at night time.
The expected behavior:
Whatever the light level is (underwater), it should stay consistent wherever the player is.
This issue existed since the Aquatic Update and is yet to be fixed.
Steps to reproduce:
Open World
Set time to day
Find a deep ocean
Go underwater one block deep, and then slowly go deeper.
You'll notice how deeper levels of the water is going slowly darker, until you go deep enough and it'll stop.
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I don't see any reason for why it should stay like this. It doesn't make any sense.

Confirmed: the illumination of blocks underwater is determined first by the sky light level at the player's position. The block light level of the blocks themselves is only visible if it is greater than the player's sky light level.
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That is similar to the lighting issue with chest minecarts, MCPE-22533. It also relates to the general problem with water lighting being too bright and affecting non-water blocks based on the player being in water, reported at MCPE-57701.
One thing I'd like to mention is that it doesn't get dark enough underwater even at night time. I still can clearly see the bottom of the ocean without the need of other night sources even at night time. I think it's mainly because there isn't much difference between light level 15 and 0 underwater OR it simply doesn't reach light level 0 underwater.
Reply from @unknown: that's the issue tracked at MCPE-57701.
Affects 1.17.2

Affects current Preview, 1.21.50.25

Affects version 1.21.50.26.

Affects the most recent Preview, 1.21.60.21.

Affects Preview 1.21.60.23.

This bug is still present in preview 1.21.80.25.
This bug is referenced, but not linked in the “Known Issues” section of 1.21.80.25’s Minecraft.net article.
This bug is connected to MCPЕ-57701, which details how a sort of unlisted night-vision effect is granted to the player in Bedrock Edition anytime they are under water.
With this bug, elevation affects the intensity of this night-vision effect.
This bug should be considered a parity issue, as this behavior does not exist on Java Edition.
I think it is not a bug, I think this is supposed to be like this