Folks,
The ArcHydro9 help is delivered as compiled HTML and can be accessed from the toolbar or can be directly opened from this location following a default installation:
[INDENT]C:\Program Files\ESRI\Water Utils\bin\ARCHYDRO.chm[/INDENT]
Additionally, from the ESRI maintained ftp site the /ArcHydro/Doc/ArcHydro1_4 directory holds the following PDF all published July 2010:
Arc Hydro Tools 1.4 - Overview.pdf (17 pg.)
Arc Hydro Tools 1.4 - Tutorial.pdf (142 pg.)
ArcHydro GP Tools 1.4 - Tutorial.pdf (177 pg.)
All fairly well written and covering pretty completely all facets of the ArcHydro tools. Please access the site, download them, and have a close read.
I think you'll find that between these resources, this is really one of the better documented ESRI extensions!
Expect that as the version 2.0 for ArcGIS 10 is further developed its rudimentary documentation will be brought up to the same high standard.
Stuart