Eddie Lack stopped 28 shots for the Canucks, who lost their second straight on the road.
Hertl opened the scoring at 10:37 of the first period, redirecting a shot by Scott Hannan. Boyle's goal came two minutes later. Thornton scored at 11:21 of the second period directly after Couture won a faceoff during a power play. Couture added a power-play goal with a minute left in the period.
Hertl, who has a team-leading three goals in the preseason, scored his second goal at 2:25 of the third.
"There's great competition," Sharks coach Todd McLellan said. "It can't be a comfortable camp for everyone. We can't keep all of these guys."