Congrats -
I had a great week and a half on the Sky Dancer a couple years ago - we went in March, and I was wearing a 5mm suit, and I think that was the best choice for myself. There were a couple dives where we hit some colder currents and on those, I wouldn't have minded a bit more neoprene, but those parts didn't last long. If nothing else, they will probably have some shorties you could throw on over your suit. You might even want to take a hood; my wife wore one on a couple dives and it helped out a lot.
I would also suggest to your DMs that taking a panga out for a champagne at Darwin's Arch is a great idea.