LA VENTANA, Mexico — In the turquoise waters off La Ventana, a sleepy coastal town on Mexico’s Baja peninsula, Claudio Rios, 41, monitored the radio of a fishing boat. It was late March. His eyes scanned the horizon, one hand loose on the wheel as the Sea of Cortez swayed beneath him. “It smells like orcas,” he said: pungent and oily. He waited for a dorsal fin to slice through the waves. For captains to chatter over the radio. For boat engines to hum together. So far, nothing. Tourists sat in boats nearby, eyes wide, hoping to spot one of several male orcas named for Aztec gods and emperors: Moctezuma, Cuitláhuac, Tlaloc. Some humans already had their wet suits on, fins at their feet — prepared to drop into the open water with the ocean’s apex predator. Swimming with killer whales is one of the rarest wildlife encounters on Earth, done in only two places: La Ventana, Mexico, and Skjervoy, Norway.…