Just need to put some detergent in the water to help break the surface tension. I have seen fine gold on a Gemini shaker table floating on the surface at a small alluvial mining operation that I used to work for, we used to add detergent to help recovery of fine gold on the table.
On the other hand, I remember going to Sovereign Hill many moons ago, and watched a father and son pan for gold, the kid said "daddy, look at all the floating gold", turns out someone threw a pan full of brass shavings in the panning area!