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.
EDIT: This issue has been fixed in preview version 1.21.60.24 and should be closed as "Fixed" and not "Works as Intended".
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Can confirm. This bug is caused by MCPE-187270. Your report is not a duplicate since it specifically describes the swamp issue.
Warm oceans are also affected, as they used to have a water_surface_transparency of 0.55, but now they don't.

This behavior is consistent with Java and Bedrock for the swamp water. This may be WAI for the water, but not for the water fog .

I actually been wanting this water transparency for the swamp similar to Java for the longest time since you cannot see any decoration blocks or entities underwater because it's too opaque when looking at it from the surface. I hope this is just a case of them bringing the swamp water transparency in parity with Java but forgot or decided not to include it in the change log.

Thank you for your report!
However, this issue has been closed as Working as Intended (see MCPE-63460).
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