The Sharkbanz deterrent field is approx. 1-2 meters (about 3-6 ft). The unpleasant sensation becomes very intense for a shark at about 1 meter (3 feet), and the field strength increases exponentially each cm/inch closer to the source due to the laws of magnetism.
The electromagnetic field is increased by your movement, but purposeful movement is not necessary. The simple rhythmic current of the ocean moving you around is enough.