If you are still getting a message stating that the Arc Hydro Tools are already installed, but you do not see them in the Add/Remove Programs window and the Arc Hydro directory has been removed, you need to manually delete the Arc Hydro installation key in your registry as follows:
1. Click Start and type regedit in the Run window. Browse to the key:
HKEY_LOCAL_MACHINE\SOFTWARE\ESRI\Applications\WaterUtilities\ArcHydro
or
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\ESRI\Applications\WaterUtilities\ArcHydro
2. Right-click this key and select Delete.
The Arc Hydro tools are installed by default under:
C:\Program Files (x86)\ESRI\WaterUtils\ArcHydro
Or
C:\Program Files\ESRI\WaterUtils\ArcHydro.
Christine Dartiguenave
Esri Water Resources Team