I think that you're being confused unnecessarily about jellyfish. I suspect that what is being referred to are the "sea Lice" infestations that can occur during season. Luckily for you, your trip is outside of that season, which is March to July, peaking in April & May. Outside of this, Coz does not have a jellyfish problem.
You can read about the condition and its prevention & treatment by searching our board archives. Here's one such post--->
http://www.scubaboard.com/showthread.php?s=&threadid=18954&highlight=sea+lice
As regards, Sea Safe, you can read about & order it here---->
http://www.nidaria.com/
Concerning what you need in the way of thermal protection, such comfort is rather idiosyncratic. To put it in perspective, below is a piece, in part, from my June '00 "Ask RSD" column in "Rodale's Scuba Diving":
"Have you ever been out on the street in the spring or fall and noticed people wearing everything from shorts & T-shirts to full coats with hats & scarves, all apparently in comfort? Well, things are no different underwater. This comfort is highly variable among individuals depending on such factors as percentage of body fat, age & tolerance to thermal discomfort. Other determinants, like how many dives a day over how many days you are planning, are also important. When you ask "What wetsuit should I wear?, you'll get answers ranging all the way from "just a bathing suit" through "5 mm full."
The question is better asked: "What water temps can I anticipate at my destination?" You'll receive a pretty accurate answer to this, and then can decide what's best for you based on your personal thermal comfort needs.
It's always a good idea to come thermally prepared, just in case."
That having been said, expect Coz water temps in July of ~82 degrees plus/minus a degree. Personally, if was doing multiple dives over multiple days at that temp, I'd take a 3mm full. If you get a bit too warm during a dive, you can always let some water into the suit. Much harder to warm up. Whatever you decide, make sure that you at least wear a full dive skin. As others have mentioned, it's good environmental protection.
Have a great time.
DocVikingo