I know this is an old post, but thought I would add some thoughts to this.
Some beaches in Queensland use shark nets. This obviously only protects you from sharks. You do not tend to find sea snakes at all, as they are rather shy.
The tropics of Queensland are rather different. It depends on when you wish to swim. Dry season and non stinger season is May-October and is safer time re not getting stung. You do not find the larger sharks in the tropics but only reef sharks- which are no harm and are smaller. HoweverI have never seen them except at a reef once in the shallows. rare and they do not bother you.
You must not swim in the wet season as there are irikandji which are tiny and can kill you. Box jellyfish also . The wet season is October- end of May approx.
There is a net at Port Douglas, but these can get hrough and it is taken out in the wet as the water gets rougher.
What you will find find in the water closer to the beach in the far orth or Queensland are crocodiles, so I dont swim at all at that time of year.
All the beaches that are safe for swimming have signs of what to watch out for. Councils have become very wary of this and especially tourists who have no idea about our beaches and hence we have warning signs and only swim between the flags.
So, really I would only swim near the beach. It also is a safety thing as if you are alone then noone can help you. Some beaches are partolled by lifeguards, however a lone swimmer will not be seen.