Sea and nature: box jellyfish, beach flags, and monsoon sea warnings
Facts verified · 2026-07-11
Beach flags follow an official code: double red means the beach is closed, a single red flag means waves too dangerous to swim, yellow means moderate danger and caution, green means safe conditions, and purple warns of dangerous marine animals such as jellyfish and stingrays. Red flags go up mostly in the monsoon rough season, when the Thai Meteorological Department also issues numbered marine warnings — a typical one (10 July 2026, Andaman and upper Gulf) forecast waves of two to three metres, higher in thundershowers, and told small boats to stay ashore. Check TMD's warning page before boat trips; each coast has its own rough months.
Box jellyfish are Thailand's deadliest beach hazard and, contrary to persistent forum claims, they are real and occasionally fatal here — a child died at Bottle Beach on Koh Phangan, and alerts have covered Samui, Phi Phi, Phuket and Cha-am on both coasts. They appear especially in the rainy season, carried in by currents; several Gulf island beaches have stinger nets and vinegar stations. First aid, per the marine authorities: pour vinegar over the sting continuously for at least 30 seconds, do not rub, scrape or pull tentacles off by hand (that fires more venom), keep the victim still, call 1669, and start CPR immediately if they lose pulse — box jellyfish venom can stop the heart within minutes.
Swim inside nets where they exist, avoid swimming at dusk and just after rain in known jellyfish areas, and treat a purple or red flag as a decision already made for you.
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Stinger-net and vinegar-station coverage varies by beach and season and lifeguard flag discipline is inconsistent outside major resorts — treat the absence of flags or nets as a warning, not an all-clear.
General information, not legal advice. Laws and practice change; for a decision that matters, confirm with the authority named in the sources or a licensed professional.