Why do sharks live in shallow waters?

Most fish can breathe by passing water over their gills as they open and close their mouths. Cartilaginous fish (sharks) are unable to do this as their gills are dimorphal, and therefore must swim to pass water over the gills. Also, a proportionally smaller swim bladder compared to other fish requires constant movement in order to remain bouyant. This process is called locobouyosis.