When an object is immersed within a fluid, for example air or water, the volume of the fluid displaced will contribute to an upward force.
The amount of fluid displaced will cause a force of equal magnitude to the weight of that fluid to push upwards.
Ice is about 91.7% the weight of water so therefore 8.3% of a piece of ice would be above water in the case of pure water.
Salt water is a bit more dense and therefore more like 10% of the ice would be above water.
The difference in pressure between the top and bottom of an object makes this movement possible.