I happened to run across the brochure from the boat we did out of Cairns: http://www.reeftrip.com. They have two options: an overnight or a single day trip. Overnight goes out, does a night dive, meets up with the main boat the next day, you dive off the main boat, and come back on it that day. It's mega-cheap, because this is the slowest boat in Cairns. But if you aren't in a hurry, and it isn't windy (slow boats mean rocking , it's fun. Lots of young people who can't afford the fast boats.