Yes, indeedy...colonics are a great way to help rehydrate! When waste enters the colon from the small intestines it is in a liquid state. One of the colons functions is to draw water out of the waste material...providing water for the body and forming the stool.
When I was performing colonics I, too, would occasionally have people 'complain' about gaining weight (couple of pounds)...even when they had released a whole bunch of waste. This was common for people that were chronically dehydrated. This is also why some people don't experience much of a release for the first colonic or two...they are so dried out that everything needs to rehydrate before things start moving. In my experience, people who were already well hydrated did not experience this weight gain and had very good releases during their colonics.
I also believe that the rehydration is one of the reasons that so many people feel better after a colonic.