Hi Sue -
I emailed you a while ago (about Bali I think) - and it's amazing that you seem to be going exactly where my husband and I went last year for our honeymoon (but we did it a bit more quickly!) Like Gay and Jeremy, I went out on the Barrier Reef from Cairns and also had a fear of water - in fact I had never even jumped into a swimming pool before I was so scared! - but for some reason I decided that I was going to go scuba diving and I just did it. It was the most amazing experience I have ever had and I really recommend it. There's nothing so satisfying as facing a fear and coming out the other side with a great memory!
Enjoy the rest of your stay in Oz!