I've been a regular (3 times a year) visitor to Puerto Galera (specifically Sabang Beach) for the last 7+ years. I've never left disappointed and it's always a sad day when I board the bangka that takes me back to Manila after two or three weeks of diving there. I've been to quite a few other places in the Philippines for diving but haven't yet come across a place that offers great diving during the day and great nightlife when the sun goes down. At this point, I will mention that some people may be put off by the entertainment scene in Sabang Beach. If you don't want to be a part of it, you can find quieter places to chill out for the evening a short walk down the beach to Big/Small La Laguna beaches.
The diving really is top-notch. For me, the best part of diving there is that the sites are so close to Sabang that you never have much more than a 10 minute boat ride. Dives are generally all guided by a DM or instructor who will take care of the navigation and perhaps reassess the original plan if the somewhat unpredictable currents are unfavourable.
The level of difficulty is often dependant on the currents... an easy, relaxing, transcendental dive in the morning can turn into a roller coaster by afternoon when the tide changes. That being said, with all else being equal, there are dives of all difficulty levels to suit whatever kind of mood you are in.
There is no shortage of dive shops from which to choose. You mentioned Atlantis in your post and that is one of the bigger shops on Sabang Beach. There are plenty of smaller diveshops and everything in between also. Personally, I've been diving at Captain Gregg's and Mermaid Divers in the years I've been going there. I've never dived with Atlantis but their reputation is sound. I will most likely do my IDC with them next year.
To address your specific questions... Ed L. is spot on about the 3mm full length wetsuit taking care of any stinging jellyfish... That being said, the stingers in PG are not the agonizingly painful or even lethal kind, but rather those damn annoying type that you don't even see but only feel a mildly irritating sting on your face or hands. Lots of lion fish and scorpion fish around so keep your eyes open and be mindful of where your hands are.
The water temperature in December will probably be in the 24C-27C range.
Any other questions? How to get there? Where to stay? Feel free to ask... If you were there at Christmas time, I could meet you for a beer...
Hope you have a great time.
