Jacques Cousteau called the Sea of Cortez “the world’s aquarium.” That wasn’t marketing — it was underselling it. The narrow body of water between the Baja California peninsula and mainland Mexico is one of the most biologically rich marine ecosystems on the planet, and Cabo San Lucas sits right at the mouth of it.

If you’re staying at Sandos Finisterra in Cabo, you’re positioned at the exact point where the Pacific Ocean meets the Sea of Cortez. That convergence of cold nutrient-rich Pacific water and warm tropical Gulf water creates feeding conditions that attract everything from whale sharks to humpback whales to massive schools of game fish.

Cozumel is a 30-mile long island off the coast of Playa del Carmen, and it has some of the clearest water in the Caribbean. Jacques Cousteau put it on the map in the 1960s when he declared the reef system one of the best diving sites in the world. The marine life hasn’t disappointed since.

If you’re staying at Sandos Caracol or Sandos Playacar in Playa del Carmen, Cozumel is a 45-minute ferry ride or a quick Seek & Go excursion from the resort lobby. It’s the single best day trip for anyone who wants to see underwater wildlife up close.

The Mesoamerican Barrier Reef runs the length of the Riviera Maya — from Cancun south past Tulum and into Belize. It’s the second-largest reef system in the world and it starts less than a mile from the beach at most Riviera Maya resorts. That means world-class snorkeling is a walk-in from shore, not a boat expedition.

Whether you’re snorkeling off the beach at Sandos Caracol, taking a Seek & Go excursion to the reef, or doing a full day at Cozumel, here’s what you’ll see underwater.