From what i can gather,sharks are mid water to top feeders,and Tope are bottom feeders.
So the rubby dubby sack should be just below the water line hanging off the boat for sharks.
I've seen vids where they use balloons to suspend the bait up in the water.
The balloons aren't for bite indication,just to get the bait at the right depth.
The british record Porbeagle was caught using the same method.