Dear Mojang.
I find a bug in Minecraft 1.15 pre-release 2
If you disable the vanilla datapack and go on the water, a strange thing (like as a fog) appear on the screen (not in 1.14.4)
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This report duplicates MC-123029.
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Maybe the underwater fog is stored in the vanilla datapack, so if you disable the pack, the game tries to load the fog, so it just place the cave fog there.