99.99999999999999999% Safe, but put down the links and tips about avoiding crocs and box jelly fish.

If you see a sign next to the water saying "Danger. Don't swim here"... don't swim there!

Seriously, the signs are there for a reason. Read them. 

We went to Magnetic Island and all the beaches had clear warnings about jellyfish. There were informative signs about all types of jellyfish. Which months they are a danger. What they look like. How to treat the sting. etc... They even had buckets of vinegar. Some of the beaches had nets and were swept three times a day for jellyfish. If any were found, lethal or not, the beach was closed.

Wow!

South of Gladstone and you'll be free of all those nasties. Noah's Arks are another matter. Mainly bronze whalers near the beaches. Only seen one in my time at Main Beach on Nth Stradbroke Island. There were about 200 people in the water at the time. Most of the blokes were more scared of what their mates would think if they paddled in rather than the bronzie itself. It was only about three foot.