As mentioned before, sharks are fish, and therefore require movement to breathe because water has to flow through their gills so it can be converted into oxygen. Some shark are able to pump water to their gills using strong cheek muscles and wide spiracles at the side of their head, however the Great White Shark is more evolved. It developed a style of breathing called obligate ram ventilation. This means it is mandatory for the shark to be moving constantly or it will drown.
Work Cited: (2012). Shark Breathing: Buccal Pumping and Ram Ventilation. Retreived From: http://science.howstuffworks.com/environmental/life/zoology/marine-life/shark-drown1.htm