As of version 1.21.40, the old biomes_client.json is no longer loaded from the vanilla resources, in favor of the newer "client biome" files; however, the "water_surface_transparency" field that swamps and swamp hills used was not ported over, causing swamp water to render at the default transparency.

This technically brings swamp water closer in parity with Java Edition, but it was not mentioned on the changelog and is therefor probably an error.

The associated underwater fog has also not been changed, causing the water to appear "thicker" while submerged than while not.

Either the surface of the water should be reverted to match the underwater fog, or the underwater fog should be changed to match the new transparency.