So... my cat has mega-colon (enlarged colon as a result of weak muscles in the anus) and, without proper management of his diet, he runs the risk of severe consequences from an inability to defecate. The way to deal with this non-surgically is to basically keep his stool soft enough to be managed by the weakened muscles. Since many cats only get moisture from diet, rather than from drinking water, we do two things:
Dose him with lactulose (basically a sugar) which draws moisture to the intestines to soften the stool.
Add water to his food sources to ensure that there is plenty of moisture in his diet to ensure the stool stays soft.
As a note, a cat without mega-colon, but who is dehydrated often may benefit from a similar treatment. The food should be managed, of course, to avoid risks of bacterial growth or other spoilage.
Dry food can be used in either case, with water, but wet food is a superior choice since it's intended to have a high moisture content and results in a better consistency. It's also more like what they'd encounter if they weren't domesticated...