How to Predict Visibility at Your Next Dive Site

Why Visibility Matters for Divers

Whether you are a new diver logging your first 20 dives or a seasoned diver with a camera rig worth more than your car, visibility affects everything:

- Safety: low visibility makes it easier to lose your buddy, lose your way or miss a hazard. - Enjoyment: there is little point diving a wreck or reef if you cannot see it. - Photography and video: murky water turns vibrant scenes into a blurry mess.

So how do you predict it before heading out? It starts with knowing what changes it.

1. Understand What Affects Underwater Visibility

Visibility is how far you can see underwater, and it is limited mostly by particles in the water: sediment, runoff and plankton. The main things that stir them up or bring them in are below.

Swell and Wave Action

Swell is the biggest single factor for many coastal sites, and its period (the time in seconds between crests) matters as much as its height. The Bureau of Meteorology (BOM) explains that the longer the period, the more energy the waves carry, and that long-period swell slows, steepens and grows as it reaches shallow water. Long-period swell also reaches deeper: Sydney diver Michael McFadyen's rule of thumb is that surge is felt down to roughly the period squared, divided by four, in metres. That is about 16 metres for an 8-second swell and about 49 metres for a 14-second one. Surge that reaches the bottom lifts sediment and clouds the water, even when the local wind is calm.

So a long period is a warning sign, not a good sign. McFadyen notes that longer-period swell also wraps further around headlands into bays that look sheltered, and that periods over about 12 to 13 seconds will probably affect any site near the open ocean, even one well inside a bay.

Wind

BOM describes an onshore wind (blowing from the sea to the land) as making the sea surface bumpy, and an offshore wind (from the land) as leaving it smooth. Onshore wind brings chop to the entry; in McFadyen's experience, any onshore wind over 10 knots makes most open-ocean shore diving in Sydney unsafe.

Tides and Currents

In bays, harbours and estuaries the tide brings water in from the ocean on the flood and takes bay water out on the ebb. At Kurnell's Monument, inside Botany Bay, McFadyen finds visibility marginally better on an incoming tide. Where the tide runs as a current, the quiet period between flood and ebb is called slack water, and BOM notes that predicting it can be important for planning underwater activities. The best tide varies by site, so check local advice (examples below).

Rainfall and Runoff

Heavy rain washes sediment off the land in stormwater runoff, and it hits enclosed waters hardest. In McFadyen's Sydney experience, rain rarely affects visibility in the open ocean unless the rivers flood, but more than 100 mm over four days will certainly give poor visibility in Port Hacking and possibly Botany Bay. Water quality is a separate question: NSW Beachwatch says rainfall is the main source of pollution in swimming waters and, at its well-graded sites, advises staying out during and up to a day after heavy rain at ocean beaches, and up to three days at estuary sites.

Plankton

Growth of phytoplankton can also make open water turbid, independent of swell and rain.

2. Use These Tools to Predict Visibility

Marine forecasts

Start with BOM's coastal waters and local waters forecasts, which give wind, sea and swell, and its interactive wave maps, which show swell height, direction and period. Check its warnings too: BOM's advice is that if a warning covers your area, change your plans.

When you read the forecast, look for:

- Swell direction compared with the way your site faces (BOM gives directions as where the swell comes from). - Swell period, remembering that long periods reach deeper and wrap into bays. - Wind direction and strength, preferring light or offshore winds.

Viz Check

Viz Check is our live visibility forecast for 187 Australian dive sites. It combines swell, wind, rainfall and tide with recent reports from divers and dive shops, and shows a forecast in metres for the next 24 hours. It forecasts visibility, not safety, so a good number does not mean the entry is safe.

Local dive reports

The best evidence is someone who was in the water today. Our latest visibility reports page lists recent in-water reports from dive shops and divers, and many shops post their own conditions reports: Abyss, for example, publishes a site-by-site Sydney conditions page.

3. Match Dive Site Geography with Conditions

Some dive sites handle bad conditions better than others, and geography is your ally when picking the right spot.

Protected Bays and Coves

If the ocean swell is up, look for a site with a headland between it and the swell. In Sydney, McFadyen lists Shelly Beach and Fairy Bower in Manly among the few open-coast shore dives that stay diveable in southerly seas, and harbour sites such as Camp Cove, Parsley Bay and Clifton Gardens as normally diveable in almost any weather. Even sheltered sites have limits: a long-period southerly swell can still wrap into Shelly Beach.

Sites Away from River Mouths and Stormwater

After heavy rain, avoid sites near creek and river mouths and enclosed bays, which collect the runoff, and favour open-ocean sites, which McFadyen finds are rarely affected unless the rivers are in flood.

4. Time Your Dive Strategically

- Use the tide. At current-swept sites, dive the slack. Abyss plans Shiprock in Port Hacking around high slack, entering 10 to 20 minutes before high tide, and warns against using a general Sydney tide time; Pro Dive says to dive The Steps at Kurnell at slack water. - Watch the timing of wind changes. "In the afternoon" in a BOM coastal forecast means 1 pm to 9 pm, so if an onshore change is forecast for the afternoon, a morning dive can beat it. - Wait after heavy rain, especially in bays and estuaries, using the rainfall guide above.

5. Read the Water Before You Enter

Even with the best planning, conditions can change. Look at the water from shore before you gear up:

- Water colour: green or brown water usually means more particles; blue water is usually clearer. - Breaking waves: more white water means more churn and more surge at the entry. - Watch the sets: BOM says sets can arrive every few minutes or every half hour or more, so a long lull can make the sea look deceptively calm. Watch long enough to see several sets.

If the water in front of you does not match the forecast, believe the water.

6. Use a Dive Log to Track and Predict Future Conditions

Smart divers keep track of visibility in their logs. Over time, patterns emerge for each site. Record:

- Tide time and height - Wind speed and direction - Swell height, period and direction - Rainfall in the last 48 hours - Entry and exit times - Your actual visibility estimate

Then share that last number: adding your report to the site's page on Viz Check helps the forecast for the next diver.

7. Ask the Locals

The best intel often comes from the people filling tanks or guiding at the same sites every week. Local dive shops know when the visibility turns and which sites are better on which tides, and many post visibility reports on their websites or social media. Follow them.

8. Adjust Your Plan for Low-Visibility Dives

Even with the best planning, sometimes you will hit a pea-souper. If that happens:

- Use a torch: it improves contrast and helps you spot critters and your buddy. - Stay close: keep in touch with your buddy and agree a lost-buddy plan before you descend. - Carry a surface marker buoy, especially on shore dives in low visibility and current. - Dive conservatively: reduce depth, communicate more and check your gauge more often, or call the dive. See when to say no to a dive.

Checklist Before Your Dive

- Check the BOM forecast: swell height, period and direction, and wind - Check BOM warnings - Check rainfall over the past 48 hours - Check Viz Check and recent shop and diver reports - Pick a site sheltered from the forecast swell and wind, and a plan B - Check the tide, and the slack window at current-swept sites - Bring a torch, just in case

Predicting underwater visibility will never be a perfect science, but with the right tools, local knowledge and a keen eye on conditions, you can stack the odds in your favour. For sites that look good right now, see Viz Check's "Where to dive this week" list.

Sources

- Bureau of Meteorology: Waves and swell - Bureau of Meteorology: Tides and sea level - Bureau of Meteorology: Marine weather hazards and warnings - Bureau of Meteorology: Water sports safety - Bureau of Meteorology: Coasts and oceans - Michael McFadyen: How weather affects conditions for diving in Sydney - Michael McFadyen: Wave period and diving and boating - Michael McFadyen: The Monuments - NSW Department of Climate Change, Energy, the Environment and Water: State of the beaches 2023-24, statewide summary (Beachwatch) - Wikipedia: Turbidity - Wikipedia: Underwater vision (visibility) - Abyss Scuba Diving: Sydney diving conditions today - Pro Dive: Kurnell dive sites - ScubaDownUnder: Viz Check - ScubaDownUnder: Latest dive visibility reports

Header photo: A Reef Life Survey diver with a camera, slate and transect tape on the reef at Pearson Island, South Australia, by Peter Southwood, CC BY-SA 3.0, via Wikimedia Commons.