Download the ArcGIS 10.2.1 & 10.2.2 for Desktop Hydrology Tools Ready-to-use-Services Patch. The Ready-to-use Services in ArcGIS Online did not previously offer the Watershed and Trace Downstream Hydrology Geoprocessing tools. This patch adds the Hydrology tools to Ready-to-use Services, which you access from the ArcCatalog window.
For more information about the Online Hydrology Tools, please read our Official Release! blog.