A spring storm had retreated inland during the night, leaving a canopy of unbroken clouds over Mission Bay. About 20 engineering students and others gathered in the morning chill to launch a cockeyed-looking vessel, mechanical guts fully visible, into the still water. The contraption’s hull, cannibalized from a 16-foot, decades-old catamaran, supported a conveyor belt, motor and array of batteries...

Just a few months ago, the movement to ban single-use plastic seemed somewhat unstoppable. By the end of 2019, nine states—California, Hawaii, New York, Connecticut, Delaware, Maine, Oregon, Vermont, and, most recently, Washington—had enacted statewide bans on single-use plastic bags. Several major cities including Seattle, San Francisco, and Washington, D.C., had banned plastic straws. And statewide bans of other single-use plastics, such as plastic utensils, Styrofoam packaging,...