WaterGEMS allows you work inside of ArcGIS and includes Darwin Calibrator, Darwin Designer and Skelebrator. WaterCAD cannot be integrated with ArcGIS and the Calibrator, Designer and Skelebrator features are optional, at an extra cost. Beyond this, equal versions (ie WaterCAD 08.11.00.30 and WaterGEMS 08.11.00.30) are identical.