CASPAR BEACH, Calif. — A welding hammer strapped to her wrist, Joy Hollenback slipped on blue fins and swam into the churning, chilly Pacific surf one fall morning to do her part to save Northern California’s vanishing kelp forests. “It’s ecologically sanctioned mayhem.” Scientific diver Morgan Murphy-Cannella swims past bull kelp as she surveys a reforestation project, Friday, Sept. 29, …