Rain arrives on the coast with a certain theatrical confidence. It comes in sideways off the sea, rattles the sash windows of the guesthouse, and turns the promenade into a long grey ribbon of shine. But a wet day in a seaside town is not a wasted day. It is simply a different sort of day — one that rewards a little forward planning and a willingness to swap the deckchair for the museum ticket desk.

Rethink the Plan Before You Abandon It

Coastal rain is usually showery rather than set, which means the day is rarely a write-off. Check the rainfall radar first thing in the morning, then look for the gaps rather than the downpours. On most wet days you can find two or three drier windows of an hour or more, often in the late afternoon when the land has warmed and the showers have pushed inland.

Build the day around those windows. Put the indoor things first, while the rain is at its heaviest, then keep the beach as a reward for later. If the forecast shifts, shift with it — nothing is lost by swapping morning and afternoon. The trick is to stop treating the weather as an obstacle and start treating it as a timetable.

Head for the Small Independent Museums

Britain's seaside towns are wonderfully well supplied with tiny, eccentric, volunteer-run museums, and a rainy day is exactly what they were made for. Look out for harbour heritage centres, shipwreck and smuggling collections, old lifeboat houses, preserved fishermen's cottages, pier railway museums and lighthouses you can actually climb. Entry is usually somewhere between £3 and £10, and often free with a donation box by the door.

These places are small enough to do properly in an hour, and the person on the desk is almost always the best source of local knowledge you will find all week. Ask them three questions: where would you eat lunch, which beach is worth the walk, and what is open tomorrow? You will get better answers than any guidebook. If you have children with you, ask whether there is a trail or quiz sheet — most of these museums have one tucked behind the counter.

Browse a Covered Market or an Old Arcade

Market halls are the dry-day heart of many coastal towns: a fishmonger, a greengrocer, a butcher, a stall selling buttons and zip repairs, another piled with paperbacks and chipped china. Wander slowly. Buy something small — a slice of cake, a warm pasty, a paper bag of cockles with vinegar — because the traders who serve you are the same people who will tell you where the good chippy is.

Beyond the food halls, look for craft units in converted chapels and warehouses, antique centres, and second-hand bookshops where you can comfortably lose ninety minutes. A word of caution: covered markets are also where a certain amount of damp-weather tat is sold. Buy the edible and the useful, not the plastic crab on a stick.

Book a Harbour-side Café and Settle In

When a squall comes through, everybody in town has the same idea at the same moment, so ring ahead and book a table rather than joining the queue in a dripping cagoule. Ask specifically for a window seat with a view of the moored boats — most harbour cafés will hold one if you tell them you are coming.

Then commit to staying a while. Order the day's catch, a bowl of something hot, and a proper pot of tea, and let the rain do its worst outside. Two hours over lunch is entirely reasonable on a wet day, and many harbour-side places have blankets, board games or a wood burner to encourage you. Take a notebook, a pack of cards or simply the pleasure of watching the tide turn.

Read the Showers and Take the Beach Anyway

A beach in the rain is one of the great underrated pleasures of the British coast: empty, dramatic, and entirely yours. Check the tide table before you go — low water gives you firm, hard sand, which is far easier walking than shingle, and it opens up rockpools, fossils and the kind of beachcombing that vanishes at high tide.

Go for the gaps. Twenty minutes of drizzle followed by a rainbow is a fair bargain, and the light after a shower is often the best of the day. Take a flask and a towel in the car, keep an eye on the horizon for the next dark band of cloud, and turn back before it reaches you rather than after.

Dress for It and the Day Gets Longer

Most coastal discomfort is a clothing problem, not a weather problem. A few sensible things, kept permanently in the boot of the car, will buy you hours of extra time outdoors:

• A proper cagoule with a hood, worn over layers rather than under a bulky coat
• Wellies or waterproof boots, plus a spare pair of socks in a dry bag
• A flask of tea or coffee and a couple of decent snacks
• A small towel for wet benches, wet dogs and wet children
• A change of shoes and a dry jumper to leave in the car

Hang wet gear on the radiator overnight and it will be ready again by morning. Do that, and a rainy day stops being something to sit out and becomes another version of the seaside — quieter, warmer, and very often the day you remember best.