AH congrats on your first shark its a big achievement if you knew how much effort and sharkless trips I put in to crack my first shark lol:pinch: I have learned a lot since and fair a lot better now when the bities are in seasonB)
If there is one piece of advice I can give you is experiment. We are constantly trying different rigs, method of rigging baits, depth fished, hook pattern etc. About your problem with your floats, try adding a large free running sinker above the float. The sinker will anchor the rig so the float and bait stays put on the surface. You can then add smaller sinkers below the float and by modifying the trace length can set the depth that you want to fish;) Even better attach sinker to mainline via a free running swivel and a foot or so of line that is lighter than you mainline. If you get snagged, you will only loose your sinker:cheer:
Don't be scared to fish the surface during the day, given that most the river is pretty murky. Big strips of eel and big livies will work a treat. The bigger the livie on the surface the better. A foot long bony bream will pull my float under for a bit, go for a swim and pop back up to the surface repeatedly. It must be like one of those big Maccas signs for sharks!
I hope to read about plenty more of your shark encounters in the future!