That hydrotherapy sounds more like a deep enema than anything. I agree that it could help constipated people who can't seem to get impactions out, especially right before they need to have a colonscopy (too much laxative prep seems to be quite horrible on people who are very backed up).
Gut, I can't remember if you said where you had your procedure done. Was it in a doctor's office?
The treatment WAS recommended by my GI specialist. He suggested this instead of the typical colonoscopy prep for the very reason you have cited. In addition, folks like myself who have excessively used laxatives to self-treat often build up a tolerance to them. This is why he recommended hydrotherapy.
You are exactly correct. It is nothing more than a deep enema, what the old folks used to call a "high colonic"! And, as Sarita stated, is similar to a colonoscopy EXCEPT that the water is the only thing travelling through the colon. In that regard, it is actually much safer than a scope which winds it's way through your insides. It's pretty hard for me to imagine how anyone could get gangrene from a deep enema, where the only thing being introduced into the body is warm water.
I went to a licensed clinic with professionally trained hydrotherapists. The clinic was also recommended to me by my GI guy. He told me he had been sending his patients there for a few years for various problems like chronic constipation, scope preps, etc. In addition, everyone who worked at the clinic, including the owners, had suffered with bowel problems of one type or another. It was a holistic clinic which offered belly massage, nutritionist on staff, supplements, etc.