The colors, in plain English
- Green — low hazard. Calm conditions, light surf, no warning currents. Safe for swimming for most people. Lifeguards still watch the rip-current channels and can pull a swimmer who's drifted.
- Yellow — medium hazard. Moderate surf or current. Knee- to waist-deep is fine; don't take small kids past the break, and stay where you can see the tower clearly.
- Red — high hazard. Strong surf, dangerous currents, or both. Stay shallow or stay out. Lifeguards make a lot of rescues on red days — even strong swimmers underestimate rip currents.
- Double red — water closed. The lifeguard captain has shut the swim zone. This is rare at KB but happens during storms, after lightning, or during marine-life events. Don't go in.
- Purple — dangerous marine life. Jellyfish, Portuguese man-of-war, sea lice (which are jellyfish larvae — see the sargassum page for the link), or stingrays in the water. Purple can fly alongside any other color — a green-and-purple combo means calm water with jellyfish.
- Black-and-white checkered — surf zone for swimmers only. No surfboards, bodyboards, or floatation. Crandon flies this when board traffic gets dense.
- Orange windsock — offshore wind warning. If you're on an inflatable, you can get blown out fast. Lifeguards will tell you to stay close to shore.
Who flies them at KB beaches
Three different agencies run the lifeguard towers at the three big beaches:
- Crandon Park — Miami-Dade County Parks lifeguards. Towers run roughly 9 AM to 5 PM in winter, later in summer. Crandon Park info ↗
- Cape Florida (Bill Baggs State Park) — Florida State Parks lifeguards at the south tip of the island. Same general schedule as Crandon. Bill Baggs info ↗
- Village of Key Biscayne beach — the Village contracts lifeguards for the public stretch in front of the residential portion. Village Public Works ↗
The Ritz-Carlton beach has its own lifeguards and its own flag policy that may not match the public beaches a few hundred yards north. Always check the closest tower to where you're going in the water — flags can differ between Crandon north and Crandon south by half a mile of coastline.
How often the flag changes
Lifeguards reassess at the start of each shift and any time conditions visibly shift — a swell rolling in, a thunderstorm building over the mainland, a wave of jellyfish coming up the channel. On a typical Sunday at Crandon the flag may switch yellow → red → yellow as the tide and wind change. Don't assume the flag you saw at 10 AM is still the call at 3 PM.
If you're unsure, walk over to the tower and ask. The lifeguards are the authoritative source and they're happy to tell you what they're watching.
Flags vs. water-quality advisories — different systems
The flag color is about surf and immediate hazards. It is not the same as a Florida Department of Health water-quality advisory, which is about bacterial contamination (mainly enterococci from runoff or sewage). Both can be in effect at once, or neither, or just one. To check the bacterial picture, see Key Biscayne water quality today. To check the seaweed picture, see the sargassum risk today.
A green flag with a DOH advisory in effect means: the surf is safe but the water has tested above the bacterial threshold. A red flag with no advisory means the opposite — clean water, dangerous conditions. They're independent calls.
If you swim outside lifeguard hours
The flags only go up when the towers are staffed. Outside posted hours — early morning, after sunset, off-season weekdays — there's no one assessing conditions or watching the water. Sunset swims and early-morning runs are part of the island's appeal, but check the surf yourself before going in, and don't swim alone in deep water.
Reporting a problem
If you see someone in trouble, dial 911 first, then flag the nearest lifeguard tower. For non-emergency reports (jellyfish bloom, dead fish, water that smells bad), the Miami-Dade Parks main line is (305) 365-4100 for Crandon and the Bill Baggs ranger station is (305) 361-0961 for Cape Florida.