Spraying her with water actually isn't such a good idea, because over time it can contribute to her being afraid of you, and as you point out, the failure is that it just stops the behavior at the time.
FYI, plastic bags are a problem for a lot of cats - it's an ingredient in the bags. The only thing to do here (because they are SO dangerous) is to never leave any out unattended.
It may help to leave out appropriate things for her to chew on. Take a look in the pet store, and get stuff for teething cats. She should be past this stage - but 4 - 6 months is just a guideline, so it's not impossible to believe she's still teething. Another thing that may really help in addition to cat chew toys for teething kitties is get a box of bendy straws (it's what we used when our cats were teething) and scatter them EVERYWHERE. They are made of a type of plastic that they can't chew apart immediately, and you'll see they're chewed up before they can chew enough to swallow any. Throw those away. Depending upon how much chewing gets done, that can be a couple of days or a couple of weeks.
With cats, changing behavior is best achieved by redirection, and then, just like with 2 - 3 year old kids, you have to not only provide the "no," you have to provide the "yes."
To say "no" or "stop it" to a cat, it is far more effective to use their own language than to squirt them with water. When you see your cat chewing on something she shouldn't be, blow a short, sharp puff of air directly in her face and say "no." Then redirect her behavior - hand her a bendy straw or something appropriate that she likes to chew on, and tell her what a good girl she is when she starts chewing on the proper stuff. After you hand her the "good" thing to chew on and praise her for using it, walk away. You don't want her to (also) learn that being bad gets her attention.
When you see her chewing on something that's OK when she does it on her own, tell her what a good girl she is and praise her to high heaven. !!
I'd also do my best to put away stuff she likes to chew on that she shouldn't. Not having plastic bags she can access out is really a must. As to the other things, removing the temptation helps too. It's the same principle of selling little blank plugs that go into sockets and installing "child proof" closers on cabinet doors to prevent young children from hurting themselves.
Hope this helps,