The shoot-the-duck can be done off-ice, with or without skates, although I'm not sure how much it helps if the problem is balance, rather than leg strength, since the problem is usually finding the exact right spot on the blade to balance on while on ice.
If the student has the correct body position--behind tucked up against thighs, rather than a partial squat--I usually have them start the shoot-the-duck with one or both hands on the ice, so they can find the correct balance point on the skate. Once they're able to maintain the position with a hand on the ice, I have them extend the leg, then raise the hands. Seems to work for a lot of kids!