It isn't clear from your posting as to whether or not you're an u/w
photographer or a fledging one who doesn't have any equipment.
I've had two live-aboard trips in an around the GBR and the Coral Sea.
For a photographer, one of the most beautiful fishes in the world's ocean
[this is obviously a subjective choice] is the Harlequin Tusk Fish which
can only be found on the GBR. They say you can see it as well off New
Caledonia but I don't recall ever seeing it there. Anyway, this
Harlequin Tusk Fish is very illusive and constantly darts all over the
place and won't stay still for a cameo portrait. It is absolutely
georgeous, coral and blue vertical stripes. Don't forget, at the GBR, the
viz isn't so great so pix tend to be cloudy. There is so much 'wash'
going on with a lot of tidal action in an around the GBR and surrounding
islands...they all have big bathtub rings around them! So you can see
how tidal it is! If you come back from the GBR without a pix of this
fabulous fish, you're a chump for sure and your friends will probably not
speak to you for a while.