Mexico's Caribbean coast (Quintana Roo) offers the calmest, clearest waters due to the Mesoamerican Barrier Reef buffering wave action. Top picks for tranquility:
Short ferry from Cancun. Shallow, turquoise, pool-calm water. White sand. Golf cart transport. Sunset views. Less frantic than Cancun hotel zone.
Island west coast is leeward. Crystal clear, minimal current. World-class snorkeling/diving right off beach. Laid-back vibe. Ferry from Playa del Carmen.
Quiet fishing town 20 min south of Cancun. Reef close to shore = calm lagoon. Protected National Marine Park. Uncrowded beach, authentic restaurants.
Delfines: Public, no hotels, wide, blue flag, calmer than hotel zone. Nizuc: Sheltered by Nizuc point/reef, very calm, near luxury resorts.
North of the ferry terminal/5th Ave, beaches widen and calm down inside gated Playacar or further north at Xcalacoco. Avoid central Mamitas (crowded, waves).
Protected bay. Famous for turtles. Very calm, shallow. Great for families/snorkeling.
Look for west-facing or reef-sheltered beaches. Avoid open east-facing stretches during winter "nortes" (cold fronts) which bring wind/chop.