When gliding through the clear water of Mexico’s Sea of Cortez on a small cruise ship, one is tempted to jump in for a swim, or better yet to slip beneath the surface for some Sea of Cortez snorkeling.f lucky, you might discover a chocolate-chip star, so named because of its conspicuous brown spots. In a few sites within the Sea of Cortez curious California sea lions may approach snorkelers for a better look. Their streamlined form and amazing maneuverability underwater put human swimmers to shame.