Two different kinds of water safety come up on almost every Cancún trip — and they're both worth being clear on before you arrive.
Drinking water
Tap water in Cancún is not safe to drink for visitors and many locals. Hotels provide bottled water, and most resort rooms have a filtered water station. Restaurants typically use purified water for ice and cooking — but if you're staying downtown or in a rental, you'll need to buy purified water (garrafones — large 20L jugs — are the most economical option and can be delivered or bought from Oxxo or similar stores).
Beach flag system
Cancún's public beaches use a standard flag system to indicate water conditions:
Green — calm, swimming permitted. Yellow — caution, moderate conditions. Red — dangerous, swimming not recommended. Black — extreme danger, no swimming.
These flags are taken seriously and staffed by lifeguards at most Hotel Zone beaches. Ignore them at your genuine peril — rip currents on the open-Caribbean-facing southern Hotel Zone beaches can be strong and change quickly.
If you get caught in a rip current
Don't swim against it — you'll exhaust yourself. Swim parallel to shore until you're out of the current's pull, then swim in diagonally. Raise an arm to signal for help if needed. This is basic ocean safety that applies everywhere but especially relevant on the Caribbean-facing Hotel Zone beaches.