Dear Alham Walid

You will require Spatial analyst and utility network Analyst  extension for delineating the watershed etc..

I further am attaching a manual you please go thought it before buying the software..

Thank you so much Mohammad Rafiq 

You can also conduct hydrological modeling by using freeware software like QGIS/GRASS.

http://www.ing.unitn.it/~grass/docs/tutorial_62_en/htdocs/esercitazione/dtm/dtm4.html

http://www.ess.co.at/WATERWARE/modelintegration.html

http://geomorphometry.org/content/watershed-modeling-rstream-grass-gis-toolbox

https://grasswiki.osgeo.org/wiki/Hydrological_Sciences

if you have ArcSWAT 2009.93, you need ArcGIS 9.3.

If you have ArcSWAT 2009.10.1, you nedd ArcGIS 10.1

Dear Alham Walid

You need have two extensions: 3D Analyst, Spatial Analyst.

ND Liem