Hi Steve
I have just returned from Christmas Island off W Australia coast. Is that the one you are referring to?
Top viz, 20 to 50 metres. All steep wall diving and the most extensive healthy hard coral diving I have done. Water was 28 degrees but I hear is it 24 degrees in winter.
I had a 12-24mm for the reef fish, a little too wide as they were always on the move. Recommend 105 for fish portraits as the viz was good enough.
My highlight was seeing a marlin.... and whale sharks.
Why would you go in July? November to February is whale shark season. And you will see the famous red crab migration in December.
I won't go on as I'm not even sure we are talking about the same Christmas Island.
Feel free to message me if you want to know anything else
HTH
Mary