British water is colder than it looks

A bright August afternoon on the Cornish coast might give you 17°C at the surface. By February, the North Sea can sit at 5°C, and the water off the east coast is colder still. Because water carries heat away from the body many times faster than air does, a swim that feels bracing in a pool becomes genuinely dangerous in the sea within minutes. Most of the difference between a lovely hour in the waves and a shivering retreat to the car park comes down to one piece of kit. Get the wetsuit right, and the British coast opens up all year round.

Thickness: pick the number for the season, not the sunshine

Wetsuit thickness is written as two figures, such as 4/3, meaning 4mm through the torso and 3mm through the limbs. Your core organs sit in the torso, so that is where the millimetres matter most.

• 3/2mm suits warm summer swims and short surfs from roughly July to early September, especially in the south west.
• 4/3mm is the great British all-rounder. It covers late spring and early autumn, and with boots, gloves and a hood it will stretch to milder winter days.
• 5/4mm is the winter workhorse for most of the UK, usually with an attached hood and 5mm boots.
• 6/5mm is for the coldest water, long sessions and anyone who feels the chill badly. Expect less flexibility and more effort to get into.

Thicker is not automatically better. A snug 4/3 that fits you properly will keep you warmer than a baggy 5/4 that lets water circulate, and you will paddle, swim and stand up far more easily in it.

Seams: where the cold sneaks in

Neoprene panels are only as warm as the joins between them. Water seeps through stitching, and once it is inside, your body has to heat it. There are three levels to look for.

• Flatlock stitching threads right through the material and leaves a line of tiny holes. Fine for summer, chilly in October.
• Glued and blindstitched seams are glued first, then stitched without punching all the way through. This is the sensible minimum for British waters.
• Taped or liquid-sealed seams add a strip or bead of flexible sealant over the join, often on the inside. These are the warmest and the most expensive.

Have a look inside the suit at the chest, shoulders and crotch, the places that take the most strain. If the seams there are taped, that is a good sign. If they are bare stitching, budget for a thermal vest underneath.

Fit beats the label on the chest

Sizing varies enormously between makers, so ignore the number on the tag and go by feel. A new wetsuit should feel snug everywhere, like a firm handshake, with no pockets of loose material at the small of your back, behind the knees or under the arms. It should not restrict your breathing or pinch your neck.

Check a few specific spots before you buy. The neck seal should sit flat against the skin without choking you. Reach both arms above your head: if the suit drags down on your shoulders, it is too short in the torso. Crouch down and check the fabric does not bunch painfully behind the knees. Neoprene softens and stretches a little once wet, so a suit that feels slightly tight in the shop is usually right, while one that feels comfortable and roomy will balloon with water once you are in.

Neoprene, linings and what you are actually paying for

Two suits of the same thickness can feel completely different. Limestone-based neoprene is lighter, stretchier and less stiff than older petroleum-based material, which makes a real difference when you are pulling a suit on in a cold car park.

Look for a smooth, single-lined panel across the chest and back. Wind chill is a genuine factor on exposed beaches, and these panels shrug it off. Inside, thermal linings such as fleece or hollow-fibre grids trap a thin layer of water against the skin and warm it quickly, letting a 4/3 do the work of a 5/4 without the stiffness. For winter, a separate hood, 5mm boots and 3mm gloves complete the system, and a thermal rash vest adds another few degrees for very little money.

Looking after it, and looking after yourself

A wetsuit lasts years if you treat it well. Rinse it in fresh water after every session, dry it inside out in the shade rather than in direct sun, and never leave it crumpled in a hot car. Store it flat or on a wide, rounded hanger, and wash it occasionally with a dedicated wetsuit cleaner rather than detergent.

The habits matter as much as the rubber. Build up your time in cold water gradually through the autumn instead of jumping straight into a January sea. Learn to expect the first gasp of cold water shock and to breathe through it. Swim or surf with others, know your exit points, and take a flask and a changing robe for afterwards. Do that, and the British coast stops being a summer-only affair. It becomes somewhere you can swim, surf and explore right through the year, with the beaches largely to yourself.