Simply type Arc Hydro in Google and you can find the following
official link easily:
http://www.aquaveo.com/downloads?tab=0#TabbedPanels
The link above provides access to the basic functions of Arc Hydro,
such as linking hydrographs to observation wells. If you want anything
more complicated than that, you will need to pay (or ask your Uni to
pay). If you want a fully functional version of Arc Hydro for free, I
am afraid this is an illeal request and should be removed from this
forum.
Please respect the developer team for their effort. More importantly,
respect yourself.
Chris