Next time a bloke gives you some old prawns and you have bait, try putting small amounts into the water at regular intervals. It's called burleying, and when the current is slow is a great time to do it. Mix the prawns up with some water, sand and maybe some tuna oil if you want in a bucket and just little amounts out every five or ten minutes.
Like the other bloke said, lose the float as well. A size two hook (either long shank or suicide design), about 60-75cm of line, then your swivel, then your sinker on the main line, after the swivel. Don't tie it, just leave it to move up and down the main line freely, a 'running' sinker rig. As the boys have said, you don't want a massive sinker, just something to give you a little bit of casting weight and enough to just get the bait to the bottom. If you burley up right, you probably won't need a big cast either, they'll be coming to find you . Best of luck mate, I really hope you can get onto some fish.
Joel