[Borek]

You have misunderstood the idea.

Both zinc/mercury amalgam and mercury sulfide are many time denser than water and they will be both sinking. However, being solids (as opposed to liquid mercury) they will be much easier to deal with and to be removed from the toilet. It is just sulfur dust that may float, not the compound after reation.

Besides, idea that these compunds will sink into the toilet making extraction harder is ridiculous - does mercury floats on the water? No, it have already sunk to the very bottom. Mercury compounds would not sink deeper.