Safe sea swimming: the complete guide

The sea is the most changeable water most of us swim in. Its depth changes through the day, it moves sideways along the coast, and the waves can be shaped by wind blowing a long way off. This guide goes through the questions to ask before a sea swim, using current advice from the RNLI and others.

Every body of water is different, so treat the questions as a guide. Wild doesn't hand out verdicts. It shows you the conditions and leaves the call to you. The questions for any swim spot are in the full pre-swim checklist.

Know your way in and out

On a beach without lifeguards, the first thing the RNLI says to know is where you can get in and out. At the coast, that answer changes with the tide. Swim England warns that the tide can significantly change your entry and exit points over the course of a 30-minute swim. The RNLI says that if you've walked round a cove or an outcrop of rocks at low tide, the water can block your way back when the tide turns.

Look at the way in, too. The RNLI's advice is to get in slowly and give your body time to get used to the cold, and never to jump or dive straight in, because that could cause cold water shock. A beach with a gentle slope gives you room to do that gradually. The RNLI notes that how steeply a beach shelves also affects the size and type of its waves. Watch the water's edge before you commit: dumping waves break powerfully in shallow water and are most common at low tide, and surging waves that don't break can knock you off your feet and drag you into deeper water.

Once you're in, keep checking on your way out. The RNLI points out that the wind can push you off course, so keep an eye on your exit point and make sure you can get back to it.

Check the forecast, then look at the water

The RNLI's advice is to check the weather forecast and conditions before any sea swim, including the Met Office inshore waters forecast, and to be ready to change your plans or cancel if the forecast isn't good. Its shorthand: if in doubt, don't go out.

When you arrive, look again. The RNLI says that if the water is too rough for swimming, don't get in, and that depending on conditions you may need to swim less or stay closer to the shore. The size and power of waves depends on how strong the wind is, how long it's been blowing and how far the waves have travelled. Waves also come in sets, and the RNLI notes that the bigger wave in the middle of a set often comes further up the beach.

Wild's beach pages put the pieces together: tide times, wave and marine forecasts, today's sea temperature, and reports from swimmers who were there.

Tides

The RNLI says tide times and heights vary through the month and can easily catch you out if you haven't checked them, and that the UK and Ireland have some of the biggest tidal ranges in the world. Spring tides have the biggest difference between high and low water. Neaps have the smallest.

Tides move water sideways as well as up and down. The Outdoor Swimming Society (OSS) explains that currents are strongest during the middle two hours of the rising and falling tide, and stronger on springs than on neaps. The slack water around high and low tide is often calmer, though OSS warns the real slack can differ from the published times. Ask whether it's safe to swim now, given the tide you'll be swimming with or against. If the answer is no or you're not sure, don't swim, or wait for a better tide.

Reading a tide table covers high and low water, springs and neaps, chart datum, and what the predictions can't tell you.

Swimming against the tide

OSS says uninterrupted tidal streams seldom flow faster than about 2 knots, and puts a world-class swimmer's sprint at around 4 knots. Swim England suggests planning your course so the current helps you: swim out against it, then let it give you a push on the way back. The RNLI's advice: stay within your depth and swim parallel to the shore.

Swimming against the tide covers route planning, how to tell if you're drifting, and what to do if the tide's winning.

Rip currents

The RNLI says the majority of its lifeguard incidents in the UK and Ireland involve rips. They can quickly drag people from the shallows out into deeper water, and they're especially powerful in bigger surf. They're also found around river mouths, estuaries, piers and groynes. Rips can be difficult to spot, but sometimes show as a channel of churning, choppy water.

If you're caught in one, the RNLI's advice is not to try to swim against it. Stay with your board if you have one, or Float to Live if you don't. If you can stand, wade rather than swim. If you can, wade or swim parallel to the shore until you're free, then head for shore. Raise your hand and shout for help.

Rip currents covers where they form, how to spot one, and the RNLI's advice in full.

Lifeguards, flags and other people in the water

When you're swimming at the coast, the RNLI says it's best to choose a lifeguarded beach and swim between the red-and-yellow flags. RNLI lifeguards patrol over 240 beaches across the UK and Channel Islands. Black-and-white chequered flags mark the area for surfboards, paddleboards and other non-powered craft, and you shouldn't swim there. A red flag means the water is dangerous, and the RNLI says not to go in under any circumstances. Lifeguard cover is seasonal, and the RNLI withdraws patrols in stages through the autumn, so check the season dates for the beach you're heading to.

Check for other people on or in the water, such as boats, jet skis, other swimmers or paddleboarders, and take care if there are or you're not sure. Make yourself easy to see. The RNLI recommends a brightly coloured swim hat and a tow float, which also gives you extra buoyancy if you need it.

Lifeguards, flags and designated swim areas explains the flags, the signs and what a designated swim area means.

Sea temperature and cold water

The RNLI defines anything below 15°C as cold water, enough to seriously affect your breathing and movement, and says average sea temperatures around the UK and Ireland are just 12°C. That makes cold water shock a real risk for most of the year, not just in winter.

The RNLI's advice is to get in slowly, splash water on your neck and face, and remember that the colder the water and the air, the less time you should spend in. A wetsuit helps you stay warm and can increase your buoyancy.

Wild's beach pages show today's sea surface temperature from Copernicus Marine satellite data. UK sea temperature by month explains how it changes through the year and why it lags behind the air, and cold water shock covers what happens in the first minute and what to do about it.

Signal, someone on shore, and who to call

The RNLI recommends always swimming with a buddy, and telling someone on shore where you're going and when you'll be back, so they can call for help if you're overdue. Carry a way of calling for help, such as a mobile phone in a waterproof pouch, and a whistle to attract attention.

In a coastal emergency in the UK, call 999 or 112 and ask for the Coastguard. If your phone shows no signal, don't panic: the RNLI says you can still try 999 or 112, because your phone will try to connect to any other network available. Who to call if it goes wrong has more on making that call.

If in doubt

If you ever feel uneasy in the water, get out straight away. The RNLI says the same: however much you've prepared and however experienced you are, if a swim doesn't feel right, there's no shame in getting out straight away, or not getting in at all. And if you do get into difficulty, Float to Live: tilt your head back with your ears under the water, relax and control your breathing, and move your hands and legs to help you stay afloat. Once your breathing is under control, call for help or swim to safety. Panic in the water has more on the breathing that gets you back.

The sea rewards a bit of homework. Check the tides and the forecast, know your way in and out, and pick a beach with lifeguards when you can. Then enjoy it.

Sources

1. RNLI, Open water swimming
2. RNLI, Beach safety
3. RNLI, Rip currents
4. RNLI, Tides
5. RNLI, Waves
6. RNLI, Cold water shock
7. RNLI, Float to Live
8. RNLI, RNLI lifeguard patrols reduce on many south west beaches for the 2023 season
9. Outdoor Swimming Society, Understanding Tides
10. Swim England, How to deal with currents while swimming outdoors

This guide is general information, not a substitute for local knowledge or training. Conditions change: check them on the day, and if in doubt, don't go in. Images on the blog are illustrations, not pictures of specific spots.