Thank you all for the great tips.
I do see the Pro Dive trips you referred to. It's a 3 day trip, including the Yongala as well as GBR, but I'm not sure what the diving is like compared with up north. Here is the writeup from their website:
"Wheeler and Davies, Two of the most spectacular reefs within the marine park are located to the east of Townsville.
A distance of approx. 50 nautical miles from the mainland to the reefs ensures excellent visibility typical of the Coral Sea.
These reefs provide a unique dive experience with a variety of scenery such as swim-throughs, underwater canyons, caves, gullies and sensational wall dives."
I was wondering why Taka, Mike Ball, Spirit of Freedom etc. were so expensive compared with some others, but I from what you're saying, I'm hearing that they actually visit different reefs than some other boats. That's too bad, cause their prices seem more than we can handle. . .