The eel is not attacking us. But my son, who is seven, thinks it might, so he’s screaming through his snorkel. We are swimming in Santa Maria Bay, a protected marine sanctuary on the southern end of Mexico’s Baja peninsula. We’ve borrowed masks and fins from the Montage Los Cabos, where we’re staying, and the payoff is immediate: just off the white-sand beach, a coral reef is filled with colorful schools of tropical fish (and one scary-looking but harmless eel).