I DESCEND beneath the water of a tropical lagoon, climb a small ladder and find myself gazing out of a glass dome at an undersea wonderland. Shoals of luminous fish glint and flicker in the rays of light which pierce the surface, while larger sea creatures hide among the tangled roots of mangrove trees.

Beyond, lurks something bigger – and probably deadlier. A sign warns of crocodiles.

It’s all tremendously exciting – a rare opportunity to come face-to-face with the treasures of the deep. But I am not in the Caribbean or some other far flung idyll, but a short hop across the Channel in the sweet French port of Boulogne-sur-Mer – home to the French National Sea Centre – Nausicaa.

Once synonymous with cross channel ferries – a place to sail into and drive straight through – this pearl of the Côte d’Opale is now more famous for life beneath the waves, rather than travelling above them.

The ferries have gone, and with them the traffic, leaving this ancient city, with its fortress and sturdy defensive walls to the locals and clued-up tourists.

Boulogne-sur-Mer was always more than just a Channel port. Its compact centre is a gem – with everything you’d expect from a small French city – beautiful churches, narrow alleyways bristling with lively cafes and tabacs, and, around a lovely square, some fabulous restaurants, majoring on seafood. And it doesn’t come much fresher. Old fashioned fishmongers and seafood stalls abound, while the Quai Gambetta, where the River Liane spills into the sea, is home to a fish market which is the stuff of fantasy of fans of fruits de la mer: trays of fresh herring, lustrous skate and soul, live lobster and crab, mountains of mussels and langoustines longer than your hand. It’s all you would expect from the main fishing port of this seafood-loving country. And if you can time your visit on a Sunday, you’re in for a special treat.

But we were in Boulogne not (just) to eat the fish but to learn more about them.

Far more than an aquarium, Nausicaa is the next best thing to an underwater voyage. The first thing to note is it is enormous. We made our way through each of the planet’s climatic zones – each designed to bring you as close to the resident sealife as possible, without getting wet. We gazed through lens-like portholes at glowing jellyfish, wondering at first whether they were real, as they drifted gracefully, lace-like tendrils waving sinuously. We came face to face with strange inhabitants of the southern oceans, leapt back as huge stingray swept past, gliding like bombers, and gawped open-mouthed at shark which powered overhead.

Best was the tropical mangrove, where we stood in domes protruding inside the aquarium itself, eyeballing its odd and beautiful inhabitants.

And, this being France, there is a great cafe, where we drank full-bodied local beer overlooking a pool of crocodiles. Surreal and fun.

More cheerful were the seals which have their own outdoor pool – glass panels affording glimpses of their aquatic prowess as they rocket through the water – a contrast to their playful flapping about on land. Loveable they may look to us, but to a fish they are laser guided torpedoes – swift and deadly.

Nausicaa is named, as any classicist will know, comes from the beautiful coast-dwelling goddess-like princess in Homer’s Odyssey, subject of his unrequited love – which all seems endearingly French. There is certainly plenty to fall in love with – and there will soon be even more to admire.

Already spruced up after a month-long makeover, it will shortly unveil a new 10,000 cubic-metre tank in which visitors can experience a voyage on the high seas, with 22,000 new inhabitants introduced, bringing the sealife population to a whopping 60,000 creatures – making it among the world’s biggest aquariums, though one with a serious ecological message.

The International Coral Reef Initiative has declared 2018 as the International Year of the Reef. While a trip to the tropics may be ambitious, a hop over the Channel offers the very next best thing. Boulogne makes a real splash – without having to splash out.

GO: Sail from Dover to Calais or Dunkirk with DFDS, with up to 54 sailings a day. dfds.co.uk

STAY: Choose a luxurious family flat just outside Boulogne at Le Canville, 3, rue de l’église 62360 Saint Léonard. There is acres of room in beautiful modern apartments in converted barns in this quiet village setting. Chose what you want for breakfast the night before and wake to find it delivered to the kitchen table.

If you re feeling energetic, there is also a swimming pool. Highly recommended

Book by calling 00 33321 302620 or go to lecanville.com

DO: Visit Nausicaaa –one of the world's premier aquariums. nausicaa.co.uk

FACTS: For more on visiting northern France, contact the Pas-de-Calais Tourism Board. northernfrance-tourism.com/

More on France with tips and details of events from uk.france.fr/en