Dave,
I really don't have that much problem with vibrations that I would want to put any additives in the water. I found that below a certain water thickness the vibratons are dampened automatically perhaps due to friction with the glass. Maybe Dave or greenglass have seen this effect? The only thing that I do add to the water sometimes is a strong dye (food coloring) when I test aluminized mirrors. It cuts down the reflectivity of the aluminum to enhance the fringe contrast.
BTY I posed my idea to the Yahoo interferometry group to get their input.