Brett,
Congrats on getting into Scuba at your age and taking to the level you have.
As a Rescue Certified Diver I think you already know the "by the books answer" which is that you should take your girlfriend to a LDS and have her do a Discover Scuba session with them. I believe you need DM rating or above to do a Discover Scuba Session.
Now even if you don't want to go by the books use a common sense approach.
Say you do just take her in the pool and she likes the playing around. What will you say when she asks you to take her somewhere else? Without a c-card where will she be able to dive? As her boyfriend you'll want to make her happy so you'll find someplace and chances are that place will have hidden dangers that a regular dive site won't (such as no other experienced divers to help if something goes wrong).
See once you say yes, it makes it harder to say no.
Don't forget that "It's so shallow nothing can happen" is the most untrue statement a diver can hear. Read the thread about the hooka diver who drown in 20 ft!