Reading the tide for safe ocean swimming
Ocean swimming is one of those sports where the people who have been doing it for thirty years make it look completely effortless, and the people who haven't make every basic mistake in the book on day one. Tide is one of the bigger of those mistakes. Pool swimmers know how to swim. They don't always know how to read water that's six metres up and six metres down twice a day, with rip currents that show up at half-tide and beaches that triple in size between cycles. A good tide chart, read properly, will keep you out of trouble more reliably than any single piece of gear.
The mid-flood is usually your best window
Halfway through the rising tide — call it two hours either side of the midpoint between low and high — is generally the most pleasant water to swim in. Surface temperature is freshening as new ocean water moves in. Rip currents that were strong at low tide are weakening. The shore break is gentler than at peak swing. Most of the year-round swim clubs at urban beaches around the world put their morning sessions roughly in this window, drifting with the cycle through the season. Look at the tide table, find the next low, add three to four hours, and that's a reasonable swim time.
Avoid the lowest of the spring lows
Spring tides happen around new and full moons and produce the biggest swings of the month. The lowest lows of the spring cycle expose channels and rock shelves that are normally underwater, but they also concentrate the rip currents into much narrower channels — and those rips run faster as a result. On a steeply-shelving beach, swimming at the bottom of a spring low can put you above water that's draining hard back to the open ocean. Pool swimmers without rip-current experience get into trouble quickly here. Either swim the mid-tide window earlier and later, or pick a sheltered tidal pool that fills from incoming swell and is safe to swim in across most of the cycle.
Tidal pools change with the tide too
Built tidal pools — Bondi Icebergs, Bronte and Mahon in Sydney, the various Cornish pools, the rock-cut basins on the New England coast — refill from incoming swell at the top of the tide and drain partially as the water leaves. At low water the pool walls are exposed, the deep end is shallower than usual, and the basin is calm. At high water the walls are low or underwater, swell can wash directly over them, and what was a calm rectangle becomes a mixing zone. For new ocean swimmers, tidal pools at low or mid-tide are the safest entry to open-water swimming. At high tide in a big swell, they can be dangerous.
Why prediction beats observation
It's tempting to walk down to the beach, look at the water, and decide whether to swim based on what you see right now. That's a much worse strategy than it sounds. The water you can see is now. The water that will be there in twenty minutes is what matters — and that's what the tide chart tells you. A beach that looks calm at the moment you arrive can be a different beach an hour later if the tide is turning. Harmonic prediction at the gauge nearest your beach gives you the next several hours with high accuracy. Use it. Decide before you change into your suit, not after you've put your foot in.
Water temperature is the other axis
Tide alone doesn't tell you whether the water is swimmable for an unprotected swim — that depends on sea surface temperature. The places listed on this site that have SST data attached will flag a swim window only when the water is above 14 °C. Below that, even strong swimmers should be in a wetsuit, and most non-acclimatised swimmers should be doing something other than getting in. The tide can be perfect and the water still too cold for sensible immersion. Read both signals together. The tide tells you when the water is right; the temperature tells you whether the water is right at all.