There is something special about doing magic with an ephemeral object, something that exists for only a short time. In this case, we're talking about ice.
Ice is a magical thing. It appears spontaneously in water, a substance essential to life. Defying gravity, it floats to the top where it shines like a diamond. If we touch it or try to hold it, it melts away, vanishing like a magician, transforming back into water again.
Floating water in it's crystalized state is the ultimate levitation effect. To a spectator, the idea of a thread attached to, or a magnet hidden in the ice is immediately ruled out, there is no proving required when the ice begins to float. Your audience will know instantly and intuitively that what they're seeing is absolutely impossible.