I went out on the Nimrod explorer at the end of last year and would recommend it. One thing worth mentioning is that if you have some flexibility with your time, and you do your research beforehand as to which trips on which liveaboards you want to do, you can make substantial savings if you wait till the last minute and book when you get to Cairns as operators will drop their prices by up to half to fill the boat up if there are any spare places left. There's always the risk that you don't get exactly what you want, or even worse, end up with nothing, though...