Yes, if it is still permitted, he would want to dive the Great Barrier Reef. I'm thinking we would fly into Melbourne for the Open, make our way to Sydney for the start of the cruise if we ultimately chose to do the cruise, and then end the trip with the diving portion before heading home. The alternative is fly into Melbourne, see the Open match and a few sights there and whatever day trips make sense from there and then head to Sydney and see the sights there and do one or two day trips (including the dive trip).
While we had not been considering the cruise option before (because we honestly hadn't thought of it), the pricing right now does make it rather attractive.