Placing a torch at the beach is lighting up the Ocean floor. The lightlevel brakes hard mosliy at the chunk borders. If you place Blocks in those areas you can light up even more ocean floor. 1.12.2 is not effected.
PS. I think I had a similar issue after a forest fire with illuminated air areas without any nearby light sources in earlier 18w snapshots
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As long as you don't experience MC-125134 this is truly Working as Intended.

@ Kumasasa
Versions 18w19a&b were unplayable for me it freezed after short time and I had to kill java via task manager.
Versions 18w20a&b were also corupting my worlds with chunkloading problems.
After deleting the corupt region file 18w20c works fine with my 18w20b world.
I think my first pictures are not spectacular enough

@unknown, didn't understand. Is this issue persising or not ? If persisting, please
recreate the world in 18w20c
provide the seed and the coordinates

New World generated seed: 12345
Found deep ocean at -2196 / 63 / -1096(optional) set time night
look at ocean ground
place torch
remove torch
look at ocean ground

Test 2 another PC fresh snapshot download
Seed 54321
Position 280 / 63 / 10
does not happen always and everywhere
best results with night + deep ocean + gravel

I forgot to say:
Iit's persistent.
It's saved.
And with blockupdates at the borders it spreads.

Hadn't the time to test (will do later), but isn't this issue MC-91136 ?

I think it's different, I know it from 1.12.2. where light updates can be forgotten, after a block update the light level is correct.
Not so in this case with, where ~20 blocks away from a torch the light level changes from 0 to 15 while the next block stays at 0.

I think this is the MC-128299 bug.

If I see the pictures of MC-128299 and its duplicates, they look very familiar to me.
Quoting from the minecraft wiki:
In 1.13, the Respiration enchantment and the potion of Water Breathing will no longer increase the
player's underwater visibility. The visibility also changes depending on the
biome the player is in,
*and water reduce light by 1 every block deep*, so only night vision is needed for better underwater visibility.
WAI.