SÁBANA DE GUACUCO, Venezuela—Desperate for water, construction worker Pedro Pirela and his neighbors recently laid a nighttime ambush. Alerted by the roar of a water truck that supplies nearby hotels, they blocked the street and forced the driver to stop, then siphoned off his precious cargo.

Mr. Pirela and his co-conspirators had no choice, they said. A nationwide water shortage is crippling Venezuela, leaving faucets dry and contributing to rolling blackouts. Here on Margarita Island, a tourist destination that is also home...