Diving and tides: slack water and the best time to dive

The short answer: dive at slack water

When the tide rises and falls, the water doesn’t just move up and down. It also moves sideways, flowing into bays and inlets on the rising tide (the flood) and back out on the falling tide (the ebb). In between there’s a lull when the current stops or nearly stops before it turns. That lull is slack water, or slack tide, and we cover it in more depth in our guide to tidal currents.

For divers, slack water has three big advantages:

• You stay where you want. With no current, you can work a reef or a wreck at your own pace and get back to where you started without fighting the water.
• You use less air. Kicking against a current is tiring, and when you’re working hard you breathe faster.
• Ascents and exits are easier. Safety stops, the anchor line, a boat pickup or climbing out over rocks are all much simpler in still water.

There’s also a safety line to keep in mind. DAN, the Divers Alert Network, advises against diving in currents stronger than 1 knot, about 1.15 mph, without proper training and experience. At an inlet or a point, the current can go from almost nothing at slack to several knots by mid-tide.

Slack water doesn’t always come at high tide

A tide table gives the times of high and low tide, when the water is at its highest and lowest. It’s tempting to assume the current stops right then. Sometimes it does, but often it doesn’t.

The Canadian Hydrographic Service explains this by separating two things people tend to mix up:

	What it is	Where you see it
Stand of the tide	The time around high or low tide when the water level barely changes	In the tide table
Slack water	The time when the current barely flows	In current predictions, where they exist

According to the service, the two don’t have to coincide, and slack doesn’t fall at the same interval from high or low tide everywhere. In the mouth of a bay, a harbor or a lagoon, for example, water can keep flowing in for an hour or more after high tide, because the basin behind is still filling. The same happens in reverse after low tide. And in a channel between two bodies of water, the current depends on the difference in water level between the two ends, not on the tide at one of them.

That’s why NOAA publishes tidal current predictions separately from tide predictions. For each current station they list the time of every slack and of the strongest flood and ebb, with speeds. If your site is near a current station, use those times. If not, the local dive shop usually knows how far slack lags the tide table. And in the water, the signs are easy to read: kelp or sea grass laid over, a tight anchor line or bubbles drifting sideways mean it’s still running.

The size of the tide matters too. On spring tides, around the new and full moon, more water moves in the same time, so currents are stronger and slack tends to be shorter. On neap tides, the opposite. Our guide to spring and neap tides shows how to tell which week you’re in.

When visibility is best

Visibility depends on what’s floating in the water: stirred-up sand and silt, fresh water from rivers, and plankton. The tide changes where the water in front of you comes from:

• On the rising tide and at high tide, cleaner offshore water usually flows in. You notice it most in inlets, harbors, estuaries and lagoons, where the end of the flood is often the best window.
• On the falling tide, water drains out of the bay or estuary carrying sediment and fresh water, and visibility drops.

Blue Heron Bridge in Riviera Beach, Florida, one of the best-known shore dives in the country, is a textbook case. According to an article in DAN’s Alert Diver, diving there revolves around slack high tide, which brings the gentlest current and the best visibility, typically 20 to 40 feet. About an hour after high tide, visibility drops significantly and the current picks back up.

Other factors can matter as much as the tide, or more:

• Rain and runoff. After a few days of heavy rain, rivers and storm drains carry mud into the sea, and near river mouths the water can take days to clear.
• Storms and swell. Big waves stir up the bottom, and long-period swell stirs it deeper than short wind chop.
• Plankton. In spring, plankton blooms along many temperate coasts and turn the water green and cloudy even when the sea is flat.

Points, channels and inlets: where the water runs

Tidal currents speed up wherever the water has less room to pass, like a river running through a narrow gorge. For divers, the places with the most current are:

• points and headlands, where the flow wraps around the land and forms eddies behind it;
• channels between islands, or between an island and the mainland;
• inlets to bays, harbors and lagoons;
• pinnacles and ledges that rise from the bottom in the middle of the flow.

These spots are rich in life because the current brings food, and they’re also the least forgiving of a timing mistake.

DAN warns about two effects that surprise many divers. When a current hits a wall or a pinnacle, it can push you down; in other places the water wells up and drags you toward the surface. If it happens, try to stop the movement as quickly as you can, holding on to rock if you need to and avoiding touching marine life. If you’re being pushed down, keep equalizing; if you’re being pushed up, keep exhaling and don’t hold your breath.

Where the tide is very small, like much of the Gulf of Mexico or the Mediterranean, there’s little slack to wait for. Currents there are driven mostly by wind and ocean circulation, so the tide table matters less than the wind, the swell and what the dive shop tells you that morning. Our guide to seas with almost no tide explains why.

Drift diving: using the current

Sometimes you don’t avoid the current, you ride it. On a drift dive you let the water carry you along a reef or a wall, and the boat follows your marker and picks you up wherever you finish. You barely kick, so it can be very relaxing, but it takes different preparation:

• Training and a good briefing. DAN recommends learning drift diving from an instructor and having the divemaster brief you on that site’s current before you get in.
• A surface marker buoy (SMB) and a reel, at minimum. Many divers also carry a whistle and a signal mirror. The SMB is what the boat follows.
• Stay together. Stick with the group or at least your buddy. If the two of you get separated from the group, stay together.
• Don’t fight the water. If the current sweeps you off course, moving a few feet to one side, up or down can sometimes ease its effect.

When it isn’t a drift dive, say from shore where nobody picks you up, DAN’s rule is the opposite: start against the current and come back with it, and split your air in thirds, one third out, one third back and one third in reserve.

Swell and surge: the other clock

You can nail slack water and still have a bad day because of the waves. There are two things to check:

• Swell is waves that arrive from a distant storm. The longer its period, meaning the seconds between one wave and the next, the deeper it moves the water. A big, long-period swell is felt on the bottom as surge: it rocks you back and forth, kicks up sand and can push you into the rocks. On the open Pacific coast, at sites like La Jolla or Monterey, check the swell before anything else.
• Surface waves matter most for getting in and out. A rocky exit that’s easy at low tide can get tricky at high tide with waves breaking over it, or the other way around. Watch how they break and pick your exit before you get in.

Offshore wind deserves a special mention: it flattens the sea near shore, but it pushes anyone at the surface away from the beach, whether they’re snorkeling or waiting for a buddy.

What TidePlanet shows on the Diving tab

Every coastal place with a wave forecast has a Diving tab. Here’s what you’ll see and where it comes from:

• A rating for the day from 0 to 10 and the best window to dive, a stretch of 40 minutes to 3 hours. We score every 10 minutes for swell and its period, surface waves, wind, current, tide and daylight.
• Today’s slack water, shown as purple bands over the day’s tide. We work it out from the tide curve: it’s the time around each high and low tide when the water level changes slowly, at least 15 minutes either side. In other words, it’s the stand of the tide, not a current prediction, which is why the page warns that in inlets, channels and between islands the water can keep running for an hour or two afterward. Where the tide is tiny, there’s no slack and it doesn’t count toward the rating.
• The current, from an ocean model (Copernicus Marine and Météo-France) with cells about 5 miles (8 km) across. It shows the trend off the coast, not the current at a particular point or channel.
• Likely visibility: good, fair or poor. It’s an estimate from the swell of the previous 48 hours, the rain of the previous 72 and the wind of the previous 24, not a measurement, so it knows nothing about plankton or nearby rivers.
• Water and suit, from today’s water temperature. Our guide to water temperature and wetsuits explains how to choose.
• Nearby dive sites, wrecks, artificial reefs and dive shops on a map and, where there’s data, how deep it gets off the shore.

Three examples. In Key Largo, the page usually marks four slack windows a day around the high and low tides, next to the reefs offshore. At La Jolla and Monterey, you’ll see the slack windows too, but on that open Pacific coast the swell and its period often weigh more in the rating.

A plan for choosing your dive time

1. Check the tide table for your site and note the high and low tides during daylight.
2. Pick slack at high tide if you can, especially near inlets, harbors or lagoons.
3. Adjust for the site. At an inlet, a channel or a point, use NOAA current predictions if there’s a station nearby, or ask the dive shop how late slack runs there.
4. Check the moon phase. On spring tides, slack is shorter and the current builds sooner.
5. Look at the swell, wind and rain of the past few days: they tell you more about visibility than the tide alone.
6. Get in a little before slack, not right on time, and plan to be out before the water starts running again.
7. Carry an SMB, fly a dive flag, always dive with a buddy and stay within your training.

If the sea doesn’t look right when you get there, don’t go in. No table replaces what you see in the water or the advice of people who dive there every day.