Yes, it’s certainly possible to lose weight without exercising. The best diet for weight loss AND health/energy is easily a dumbed down version of The Zone described in a book with an unfortunately sensationalistic title:
Don’t be fooled by the title though. The basic premise is to stabilize your blood sugar to avoid the insulin spikes from highly gycemic carbs. The insulin converts blood sugar into fat, and the amounts released from cheap carbs will be overkill, lowering your blood sugar too far, making you hungry again. It’s the worst cycle possible for weight loss. The Zone is too detailed and obsessive to be practical.
As far as Weight Watchers, I don’t recommend it. Sure, if you stick to the calorie counts you will lose weight, but doing so can be tough. With blood sugar stability you aren’t hungry in the first place.