We normally plan to dive there in December, but if you get good weather you should be fine.
At that time of year you are starting to get into cyclone season, but if you don't get one (your chances of avoiding one are reasonably good, especially in January) the diving should be great. Our "summer" in this part of Australia is either the calmest or roughest time of the the year - to the point where it is impossible time to dive. As it is the wet season you may get very heavy rain especially near shore - this may make light a bit dull on dives.
The dive operator will know in plenty of time if there is going to be a cyclone and respond accordingly. Weather predictions in that regard are very good.
Viz should be good on the outer Reef or Coral Sea, especially out at Osprey. You are very unlikely to get anything less than 30 m and often much much more in the Coral Sea.
As far as big fish go you should see lots of sharks at Osprey and maybe some potatoe cod, depending on where you dive. Maybe tuna and barracuda, and lots of smaller fish etc. Night dives are good too out at Osprey as you can see flashlight fish glowing in the reef on some dive sites.
The Cod Hole on the Ribbon reefs are where you can see big cod, and moari wrasse as well as some little sharks.
Try and do Steves Bommie on the way back if you get the chance - masses of little fish, corals, stonefish - a brilliant dive.
I have been on dive trips out in this area more than 15 times - last time was December 2005 - but can't actually recall the temp - probably around 24 C to 26 C - the Coral Sea seems cooler than the Great Barrier Reef - some people wear 3 mm shorty but 3 or 5 mm steamers are the most common suit. I wear a 3 mm long john and 5 mm hooded jacket. Get some strange looks but am by far the warmest on boat even after 4 long dives / day.
Expect the trip out to Coral Sea reefs such as Osprey to be really rough. On one trip back the sea was so calm we did a dive mid trip in open ocean under the boat at night looking for plankton / sharks - the water under the boat was 2500 m deep - but this is an exception. Usually it is rough and seasickness may be a problem unless you are an experienced blue water sailor / professional fisherman or something like that.
On one very rough trip in winter everyone on the boat, including the crew, was seasick except me. I am actually prone to get seasick but am paranoid and take lots of seasick pills. Don't be lulled into a false sense of security as you motor out to sea in the calm water inside the outer reef. Be prepared and you should be OK. Once you are out at the reefs in the Coral Sea the boat operates/anchors in the sheltered areas and the sea is generraly very calm.
It is worth going with a charter that is happy if you dive to at least 40 m - Taka is a really nice boat for example but doesn't like its clients diving deeper than 35 m - that extra 5 m is useful in the Coral Sea - not usually significant on the GBR.
There is some great diving on the outer reef and Coral Sea - much better than the day trips you can do out of Cairns or Port Douglas.
Have fun.
