What decides go/no-go is
today's sea
Today's sea decides, not the season.
The variable that decides whether you can dive differs by region — in the West/South Sea it's tides (tidal cycle/current), in the East Sea/Ulleung/Dokdo it's swell and high seas, and for every offshore boat it's high-seas warnings and wave height. Fortunately Korea's free official forecast infrastructure is dense: weather from the Korea Meteorological Administration (KMA), and tides/current/water temperature from the Korea Hydrographic and Oceanographic Agency (KHOA).
Before diving: a free 3-step official check
(1) Marine forecast for wave height and high-seas warnings, (2) tide table / current forecast for slack-tide times, (3) ferry service info for cancellations. English support is limited, so capture and translate in advance.
weather.go.kr — wind direction/speed, wave height, water temperature and swell for 9 marine forecast zones + high-seas/typhoon warnings.
khoa.go.kr — tides (mulddae), currents and water temperature; tidal phase and high/low-tide times.
Real-time water temperature, wave height, wind speed and current speed from stations and buoys.
Real-time operating status, next-day service forecast and cancellations (Ulleung/Dokdo, West Sea islands).
High-seas warning = stop signal
The KMA issues high-seas, storm-surge, strong-wind and typhoon alerts by sea area in two tiers — advisory and warning. For diving and boats the key one is the high-seas alert — even an advisory effectively halts offshore boats (departure control, ferry cancellations).
- Significant wave height is the average of the largest 1/3 of waves, so the actual maximum can be 1.5–2× higher.
- Swell has no separate alert — even when the surface looks calm it can suddenly sweep over rocky shores and breakwaters (especially dangerous on the East Coast).
What to check first
Tides (tidal cycle/current) come first — a short window around slack tide. The KHOA tide table is essential.
Swell and high seas come first — Dokdo docking only with swell of 3–5 m or less.
High-seas warnings and wave height decide departure — Wangdolcho, Ulleung/Dokdo and Tongyeong offshore islands.
- Significant wave height up to 1.5–2× and the share of swell-related accidents are based on general/press sources (verify).
- English support in forecast tools is limited — capture and translate in advance.