after emigrating from Scotland I got used to Oz weather very quickly and hate the cold here in Melbourne in winter. That said all my US friends (from NY and other North East states) all laugh at me when I whine about 10 degree (celsius) temps. If you're going to be active then I doubt you'd find it too cold. Anywhere north of Brisbane though is always reasonably warm even through winter.
Port Douglas is the closest town to the Barrier Reef and has heaps of accommodation ranging from hostels to very pricey resorts. I just got back from a trip up that way and can't recommend Wavelength Charters (
http://www.wavelength.com.au/) for a reef trip highly enough. Small boat and very knowledgeable guides. I went out with a few boats because I found I absolutely loved the reef and they were by FAR the best. Avoid the huge boats at all costs - you end up spending the whole time with hundreds of other people's fins in your face and don't see the best parts of the reef.
In the tropical north the only time you can swim most places is in winter as summer is stinger season. The water in FNQ (far north Queensland) is always warm, as an example the water temp in Cairns in August is around 25 degrees (celsius). If you're anywhere in the Southern states in winter I'd advise a wetsuit unless you really liked the cold though.