FAJARDO, P.R. — Puerto Rico, dealt only a glancing blow by Hurricane Irma, has become an impromptu emergency hub for its Caribbean neighbors that were left devastated by the storm’s wrath. Puerto Ricans relieved at being spared the worst destruction donated water, clothing, first aid and other supplies, and dozens of recreational boaters sailed to nearby islands to deliver the assistance and evacuate now-homeless islanders on the return trip. A 54-foot dive boat named the Tobias cast off on Sunday morning from the marina here, the largest in the Caribbean, and made its way eastward under a searing sun to...