Cancún uses international flag systems but tourists ignore them and get swept out by undertows. Red flag means deadly conditions - locals won't even wade in. Yellow means strong swimmers only, period. Green means relatively safe but still supervise kids constantly.

Playa Delfines has vicious undertow even on green flag days - beautiful for photos, dangerous for swimming. Playa Norte on Isla Mujeres ferry ride is much calmer for families. Hotel Zone beaches vary wildly - ask lifeguards about current Sargasso seaweed conditions before letting kids near water.