The little island of Valsörarna in the middle of the Gulf of Bothnia feels like it is 1,000 miles from anywhere. The Finnish mainland lies to the east, but when I was there on a hot August day, it was just out of view behind a thin bank of clouds. West, towards Sweden, blue water sloshed lazily, bathtub-like.

Valsörarna is the outermost island of the Kvarken Archipelago, Finland’s only Natural World Heritage Site, a cluster of thousands of islands that are literally rising from the sea.

This watery world is never the same from one year to the next.

During the last ice age, the Kvarken region was covered in mile-high masses of ice; the glaciers so heavy that they forced the Earth’s crust to sink. When the glaciers rolled away some 9,000 or so years ago, the crust began to spring back — with dramatic results. Roughly 1sqkm rises from the waters every year, about 150 soccer fields of new land. If you are one of the archipelago’s current 2,500 residents, all you can do is watch as the sea moves to the west and your boat sits on freshly exposed granite.

With the shoreline in constant flux, this watery world is never the same from one year to the next.

“There was a little bay where I used to swim and fish when I was a child,” islander Roland Wiik told me. “But now it’s above the waterline and full of cat tails. Pine trees are starting to grow in there now.”

When the islands finally poked their peaks into the air, thousands of years ago, it was seal hunters from the mainland who found them first, followed by fishermen, other hunters, and finally, as the islands grew large enough, farmers and herders. As the archipelago rose, more people came. There has never been a time when people weren’t here.

Late one evening, I took a small canoe and set out for the island of Mustasaari. In summer, the sun this far north doesn’t set until nearly midnight, and a pinkish glow suffused the sea. The name Mustasaari literally translates from Finnish as ‘black island’. There are numerous islands with this name up and down the Bothnian Coast; this is an ancient word that holds more meaning than just the literal translation.

Some of the new islands crowned with forest relatively quickly. When passengers on an approaching boat saw a mass of black reflected in the often still waters of the Gulf of Bothnia, the island appeared larger than it was and closer than it might have been, its edges diffuse and fluid. These were mysterious islands, haunted islands, where the haltija or veden väki lived, the water spirits and elves. These were black islands.

The Common loon may have added to the ghostly feel. My late-night trip was specifically to sit and listen to the haunting cry of these hard-to-spot birds. I found them in a shallow bay where I built a fire, lay out on a rock and listened. As the flames crackled and I curled into my fleece, the eerie, wavering yodel of a male loon called out, and a chorus, somewhere around the edge of the bay, sang in response. I couldn’t help but shudder, and I imagined what it must have been like to sail upon a fog-shrouded black island 1,500 years ago.

In the past, the gulf would freeze solid in winter, allowing sleigh travel across the fat thumb of brackish water to trade in salted herring or seal skins with the Swedes. But today, due to climate change and constant ferry and freighter traffic, much of the gulf no longer ices over. Instead, more islands are appearing from the sea, slowly connecting to the mainland. In about 2,500 years, according to geologists, this uplift will result in a land bridge between Finland and Sweden across the Kvarken.

To understand how this constant movement impacts daily life, I climbed into a shallow-hulled motorboat with Vesa Heinonen who, along with his wife Kirsti Lehtinen, runs Vippipooki, a guiding service for bird and wildlife watchers, fishermen and school groups.

We pulled his vessel from a rusty red boathouse and slid it through the grasses into the water. Nearly every boat house in these islands is red. It used to be that the paint was a mixture of water-repellent seal blubber and a natural red from the soil, boiled together then painted onto the homes and boathouses while still hot. Today, given that seals are endangered, commercial paints keep the colour of the islands a deep red.

It can be treacherous... Our maps change every year.

Heinonen skilfully piloted the boat through the maze of islands, pointing out new rocks that weren’t there the year before. “It can be treacherous,” he said. “One year there is a route between two islands and the next year they are connected. Our maps change every year.”

This kind of constant environmental change creates a flexible and resilient human culture, with Heinonen adding, “That’s why people here invented all the varieties of shallow bottom boats.”

For centuries, fishing and seal hunting were the principal industries in the Kvarken. Life began to change in the mid-1700s with the advent of industrial ship building that increased trade around the region and even far beyond to central and southern Europe. This life on the water naturally resulted in a culture of expert boat builders.

The Kvarken Boat Museum on the southern end of the World Heritage Site is dedicated to the preservation of regional boat-making culture, and also serves as a meeting spot for folks working to preserve other local traditions, from preparing food to making clothing made from sheep’s wool. Despite – or because of – their remoteness, these deeply-rooted people strive to protect their culture and way of life.

When I arrived, a group of women outside was frying up a local delicacy, muikku, a small freshwater fish that thrives in the brackish waters of the gulf. They were cleaning and then rolling them in a breading made of rye flour, salt and pepper. “Always butter!” one of the women called out. “Never cook them in oil. It will ruin them!”

It was a stormy afternoon and I quickly ducked out of the rain and into the museum. While I was looking at a 1950s-era picture of a handsome youth constructing a boat at the edge of the sea, a man came up next to me, pointed at the picture and said, “That’s me.” Bror Antus is co-owner of the privately held museum.

“We’re trying to preserve the boat-making heritage of the islands – and in fact of this whole stretch of the coast,” he told me. “You see, for each and every task – seal hunting, fishing, trading – our ancestors built different craft to meet what the work required.”

Antus guided me through the museum, explaining how one kind of boat was for short, shallow water coastal trading while another was for seal hunting. A third type was for trading to Sweden and a fourth for catching a certain type of fish.

“We developed such skill at constructing these craft, and I don’t want that lost,” he said.

And skill they indeed needed to live in this watery world. Although ferries now connect Valsörarna with Sweden; the massive Replot bridge connects much of the Kvarken with the Finnish mainland; and a spider web of roads covers the largest islands in the archipelago, still many places are simply unreachable except by small, personal watercraft.

Take advantage of what you can. It’s the only way to survive.

As local boat captain Jussi Mendelin told me: “You have to be adaptable out here. Take advantage of what you can. It’s the only way to survive.”

Because the Kvarken remakes itself literally every day – and forces its inhabitants to do the same.

Join over three million BBC Travel fans by liking us on Facebook, or follow us on Twitter and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter called "If You Only Read 6 Things This Week". A handpicked selection of stories from BBC Future, Earth, Culture, Capital and Travel, delivered to your inbox every Friday.