Don't hitch-hike. Especially alone as a young woman. Wolf Creek might be just a movie, but it got inspiration from real events.

Crocodiles have been spotted as far south as the Mary River near Tiaro (Just south of Hervey Bay). Don't swim in estuaries or creeks North of Noosa. I'd avoid swimming in creeks and estuaries altogether, there are plenty of better swimming spots.

Sharks are no different here than anywhere else in the world. If you would swim in Miami without fear of sharks, you should feel the same here.

There are plenty of backpackers to choose from in Cairns, Gilligan's is the one that comes to mind off the top of my head, but I'm sure there are other better ones. There are so many places to see around Cairns, seriously go over google maps and find all the waterfalls and natural wonders to explore. You can't miss out on seeing the Boulders in Babinda, or the Tully Falls white water rafting, those two are a must.

Magnum's at Airlie Beach is the better hostel of the main ones.

Palace Hostel on Bideford street is a top backpacker location in Hervey Bay. It doesn't have as good a review as the Woolshed or the other hostels but it's the closest one to the beach and the esplanade. Hervey Bay is a good laid back, sleepy spot, whale watching starts around July and finishes around September I think.

I'm a local in Hervey Bay and I've lived in Cairns for over a decade so PM me if you want further advice.