When it comes to chasing a chew toy off the end of a dock and into a pool of water, Cabin Creek Sarge takes orders from only one person: Mariah Gurule. Although legally blind, the 13-year-old Sutherlin eighth-grader senses just what kind of coaxing the 2-year-old dog needs to take his leap into the great unknown — and the cold bath that inevitably awaits him. “Before we go onto the dock, I try to get him excited,” Mariah said, adding that tug of war or a few playful swats with the toy — or bumper — usually works. “And then …