I encountered the same issue. It seems, that the path where the ArcHydroPython.pth-file has to be copied to has changed. Using C:\Program Files\ArcGIS\Pro\bin\Python\envs\arcgispro-py3\lib\site-packages instead of the proposed C:\Program Files\ArcGIS\Pro\bin\Python\Lib\site-packages fixed it for me. Use print(sys.path) within the python console of ArcGIS Pro to check which path is valid for your installation.
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