Hey mate, I was there in November. I went on a pro dive boat out of Cairns. I had just gotten my OW and went on a liveaboard. I did extensive research on this ahead of time and booked at the last minute. My advice would be as follows:
There are 3 kinds of diving from Cairns (and probably everywhere else) to the GBR. You have the shore stuff where you visit highly trafficked areas with some reef and cool stuff, the cheaper liveaboards where you stay relatively close to shore and then the pricey Mike Ball type stuff where you're way farther out. The diving quality increases as I talk about those 3 options.
I would avoid the shore diving as it will be crowded and do you really want to spend 4 hours on a crowded boat every day cruising?
I chose a Pro Dive 4 day trip. You can actually extend it at sea if you want. They'll just ferry you to another boat. The people on board were mostly younger backpacker types with a bit of everyone mixed in. Half the boat was doing their OW course. The first two days are good because only a subset of you will be doing actual diving.
The boat was nice and reasonably comfortable. I'd bring an iPod with an FM adapter as radio in Australia is TERRIBLE. Hope you like the outside as the TV is as well. Somehow they think that CSI:Miami is the equivalent of the Sopranos.
We got 4 dives in a day at different places depending on weather. We also did night dives. I did my advanced course while on board. Some of the dives were not as good as I was doing the course, but it was fun.
None of the diving was overly deep. We did a deep dive on my course, but there wasn't a lot down there, but for a massive turtle.
The trip was around $400 for the 4 days.
The boats that go farther out have better diving, but they also have currents and are not recommended for beginners. I think I'd want to do much more diving before I tried them. You'd also be looking at more money for these.
The boats have anything from bunk beds to cabins. It just depends on price.
The food was ok. There was a lot of processed meat and the like and you are sort of cattled in to the feeding process. I wans't hugely happy with this, but I sort of fit in between demographics. I'm young enough to want a boat full of fun people, but also have enough money to afford the better diving with people who are more settled and boring. I was never hungry and there are biscuits and snacks around. They'll advise you on what you need to bring.
The crew were nice and professional.
I'd suggest the following:
Screw the shore diving. Way too much effort for too little reward.
Do the closer boat diving as it is cheaper and more suited to your GF.
The couples did seem to like the trip. You have fun activities to do together and some nice scenery. Getting a decent cabin could also help.
Make SURE you go to this old guy who does a review of the different sea life there. He's some old Irish guy in Cairns, who has been doing this forever. He will help you out greatly and it's a fun thing to do the night before. ProDive included it.
The water will be warm in December. Our cook did a night dive in shorts in November. Bring sunscreen.
Have fun.
Make sense?