I would use GM top engine cleaner (TEC). Follow the directions on the can. The engine can ingest the fluid though the PCV valve hose. Then let it ingest TEC until the engine shuts down. Then let it sit the required time, then fire the car up. I would suggest not starting the car up in a garage. Also, it's good to have a new O2 sensor and plugs. When you initially start the car up you'll kill birds for miles. IMHO, I would not draw water into through the induction system. The water may shock the engine internals and break the carbon build up into chunks that will travel through your engine, whereas TEC softens the carbon into something that will flow out of the exhaust.
Another option is to run xylene/gas mixture, and that should clean the combustion system (search the board under xylene to find the right ratio of xylene to gasoline)