An amber heat health alert remains in place across the South until 5pm on Thursday, as families continue to head to the coast.
The warning comes as high temperatures continue to affect parts of the region, with health officials saying older people and those with existing health conditions are most at risk.
Beaches including Selsey, the Witterings, Bognor Regis and Littlehampton have already seen large numbers of visitors over the Bank Holiday weekend, with many people looking to cool off by the sea.
The RNLI is reminding anyone heading to the coast to choose a lifeguarded beach where possible, and to swim between the red and yellow flags. Lifeguarded beaches give swimmers the best chance of getting help quickly if conditions change or someone gets into difficulty.
The charity is also urging people to remember its “Float to Live” advice.
If you find yourself in trouble in the water, the RNLI says you should fight the instinct to panic or swim hard. Instead, tilt your head back with your ears in the water, try to relax and control your breathing.
You should then use your hands to help you stay afloat, moving them gently if needed. Once your breathing is under control and the panic has passed, you can then shout for help or swim to safety if you are able.
People are also being reminded not to enter the water after someone else without calling for help first. If you see someone in difficulty at the coast, call 999 and ask for the Coastguard.
With the warm weather expected to continue into tomorrow, families are being advised to take plenty of water, wear sun protection, avoid the hottest part of the day where possible, and keep a close eye on children near the sea.
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