I see no one mentioned Cairns Dive Center yet, and I'd say that's for good reason. I just came back from a trip with them - a two day liveaboard because that's all I had time for in Cairns.
Save your $$, go somewhere else. I was fairly disappointed.
Cattle boat, lousy food, rush, rush, rush, and worst of all, the last reef they brought us to was dead. I wouldn't have minded if they told us it was dead before we dropped in. I'd read about this in the tour books but didn't understand it until I saw it. If they made it a learning experience I would have been ok with it, but it was a supposed to be a regular dive.
Besides all that, they "hire" mostly volunteer kids for 6-9 months with the promise of teaching them to become a dive instructor from scratch. So that's who's running the show.
The wetsuits were "first come first severed" (their words) and they had more holes than good Swiss cheese. They did drop off some new ones, which arrived on the the transport boat we were leaving on. There were about 20 size 3 & 4, and 1 size 5. Since I'm a full grown adult, I needed a size 5.
The cabin was good, but we had to be out of it by 8AM - although our transport boat doesn't show up until 2PM - or we'd be charged an extra day.
As usual, they don't mention this stuff until you're out at sea.
At 50 years old, I was 10 years older than the oldest person on this boat - crew included. It became clear this dive company was for college kids who don't mind putting up with this stuff. The boat was packed with college kids and virtually no one over 30. That's why all the small-size wetsuits.
First dive: 6:00 AM. Second dive: 8:00 AM. Not my idea of fun. They ran out of some breakfast items in the galley by the time I got out of the water at 7:00AM
Also, 80CF tanks for everybody but me - I brought my own stuff which was DIN, and they had a couple of DIN tanks which happened to be 120. So I was lucky about that.
They also didn't have provisions for people who brought their own equipment, as there were no fresh water areas to rinse your stuff - I did it in the hotel room after I got off the boat. They had rinse buckets with 2" of water for wetsuit rinsing only.
Due to circumstances, I had to take this boat - take my word for it. It was the only one that fit a situation that's too long to get into. Next time, I won't take that dive buddy with me, so I'll go where I want.
So if you're a college kid and you want to dive cheaply, this is the place. If you're looking for a fantastic dive experience - everyone in Los Angeles recommended Mike Ball or Pro Dive.