Theoretically, one would think an ice cube would melt faster in salt water than it would in fresh water, as salt lowers the freezing point of water. I would think it would probably have something to do with density of the saltwater being greater than that of the ice, so it probably would interfere with the heat transfer between the salt water and the ice cube. Salt water has tons of ions floating around in it which probably serve as some type of insulating factor, which slows the melt rate of the ice cube. Fresh water has much less of these ions and therefore has less of an insulation problem, opening up the free exchange of heat from the fresh water to the frozen molecules of ice. This liberates the ice molecules from their frozen lattice structure, and they assume the liquid qualities of water from which they came in the first place.