The answer is in the video below. The author of the article, Michael Scotto, explains how you can revive a cat that has been drowned or otherwise rendered unconscious by an unknown perpetrator. In order to do this quickly and efficiently, he recommends performing mouth-to-mouth resuscitation on your cat even if they have been breathing when faced with such a catastrophe.
To perform mouth-to-mouth resuscitation on your pet during a drowning incident, place both hands over their head and prevent them from closing their eyes or opening their mouth while exhaling forcefully into its nostrils for about four seconds before slowly returning to normal breathing rhythm. This process will create enough oxygenated blood flow through the lungs so that they will be able to breathe again naturally. If necessary repeat this for another five minutes until help arrives or you are sure that there is no life left in him/her at all!