The majority of all shark attacks (bites) are classified as "hit-and-run" incidences, where a curious shark takes one bite and leaves. Most shark bites in these occurrences take place on a victim's legs, which is why if you're only wearing 1 Sharkbanz to wear the device on your preferred ankle.
For additional shark deterrent coverage, especially if you're surfing in known shark hot spots, we recommend wearing two (2) Sharkbanz units - one on your ankle and the other on your opposite wrist. The deterrent field for Sharkbanz is approximately 3-6 ft (1-2 m). The unpleasant deterrent sensation becomes intense for the shark at about 3 ft (1 m) and becomes exponentially greater each inch/cm closer to the Sharkbanz.
Depending on your height and movement in the water and on your board, wearing 1 Sharkbanz unit on your ankle could provide deterrent coverage to your forearms and wrists, but simply put, your safer wearing one on your wrist as well. In addition to reducing the risk, you'll also enjoy greater peace of mind and be able to freely focus on surfing.