If you are doing the GBR as soon as you arrive, I recommend a Minke Whale liveaboard from Cairns. I did it last summer (4 nights to Cod Hole) in the end of June/ beginning of July (middle of Minke Whale season) and it was fantastic. At least 16 whales at the same time in Lighthouse Boomie. I guess neither Mike Ball nor Spirit of Freedom go to Osprey reef while on a Minke whale trip but I can be wrong. Below some of the main points of my trip (copy/paste from a non-related thread):
- Minke whales everyday for several hours, with a peak of 16 at the same time - Dr Alastair Birtles from James Cook University, who studies minke whales was on the boat (as far as I got, there is always someone from his team in these minke whale liveaboards in June/July) and gave us presentations after dinner on the minkes and the behaviours we had observed.
- Very nice coral specially at Steve's and Lighthouse Boomies;
- Great night dives with sharks, specially at Challenger's Bay (several white tip reef sharks and at least one grey reef shark);
- Turtles everywhere;
- Large potato cods in Cod Hole as expected;
- Really enjoyed everything on the boat;
- Very scenic low level flight from Lizard Island to Cairns, going through the main highlights of the GBR.
P.S.: If you want to check the best diving Australia has to offer, do not forget the Yongala wreck.