Between Jan & March is the Wet Season In Far North Queensland, and with this is the 'Stinger Season' as there are atleast 5 different types of dangerous jellyfish/stingers in our waters. Mission Beach has (like all beaches up here) got a net that we swim in, however it does not guarantee your safety as Irukundji are tiny stingers, but potentially fatal for people if stung.
Yes Crocodiles do appear at our beaches in Far North QLD, infact we saw one at Palm Cove (upmarket beach village in Cairns) swimming about 2 weekd ago, so the lifeguards cleared the water & went to investigate. Crocs appear at our beaches this time of year as they are Territorial (males) and as there are so many of them in the estuaries & rivers, they wander into the coves and bays that make our beaches.
However Cairns & surronding areas has loads of freshwater rivers, waterfalls and cascades to swim in all year round.... beautiful places.
you can but only in netted areas ..ie where the life guards are patrolling the beach due to the stingers, and also you have crocks in the water, however some people have said that the crocks don't like moving water so they tend to stay away from the beaches.. but i personally find safer places to swim... :)