Sometimes all you need is a beach, a swim-up bar and a sun that never goes behind the clouds, but tastes are changing. Mazatlan , the scenic port city on Mexico ’s Pacific coast, has the pristine beaches vacationers crave and the real culture they will remember long after their tans fade.

Mazatlan boasts a population of 445,000 — that includes about 7,000 expatriates who got seduced by the area’s tr...