I would never let my herps roam around the house unless I was right there with them, ESPECIALLY not when I'm not at home.
And I would never, ever let them go outside!!!! A 10 foot snake, maybe, but NOT a 1.5' water dragon. You are lucky she came back, but I wouldn't risk it again. If you don't have a cage, don't have herps.
I let my boa and adult carpets roam around outside sometimes, but I am always right there in case they decide to take off.
Also, you can't make a lizard like a cat. Lizards don't work that way. You might get her to tolerate the cat eventually, and stress her out in the process. I would, however, advise you not to put your cat and water dragon together. Your cat may not seem to mind the water dragon for the moment, but who knows when your cat's instincts will kick in.