Dry suit snorkelling in Siska, Alaska: What it's like to plunge into the freezing cold waters

An expedition cruise in Alaska can be quite the adventure, especially if you're offered the change to snorkel in frigid waters.

Sensible people can do sensible things on shore excursions in Sitka, such as potter around town in search of Russian architecture, or plunder shops for fur hats. Too much Bordeaux wine and patisserie sugar, however, has addled my judgment on this Alaska cruise with suave French company Ponant. As Le Soleal sails into Sitka I find myself signing up for snorkelling in spite of glowering grey clouds and an air temperature nippy enough to require multiple layers. Oh well. This is an expedition cruise, after all, for those if not with sense, at least with a sense of adventure.

It's a nervous-looking huddle of passengers taken in hand on shore by expert snorkelling guide Ryan, whose first task is to bundle us into dry suits. "The neck is really tight, it's going to feel uncomfortable at first but, as long as you don't turn purple, you're going to be OK," he says cheerfully.

We strain and heave the rubber over our street clothes and are soon sealed off from the elements. The dry suit turns out to be surprisingly warm. The only slight contact with water is around the neck and wrists, where the cold makes it feel – unnervingly – as if seawater is trickling into the suit. Only hands and head are in contact with seawater and yet, because my core body temperature remains constant, I'm never chilled.

Encased like astronauts in our suits and rubber boots, we waddle down a pebbly beach and into grey Alaskan waters at six degrees. Green glass-like weeds grow in the shallows, replaced when the water gets deeper by kelp encrusted with snails and barnacle-like creatures. My surmise that there won't be much to see underwater in Alaska is quickly abandoned. Anemones wobble in the current, and silvery fish flit across my mask. Urchins and tubeworms cling to the rocks, crabs scuttle, the odd cod trundles past in the gloom. Sometimes sea lions might appear, though not today.

Around the fringes of rocky offshore outcrops, starfish (or sea-stars, as Ryan calls them) are flamboyant in purple and orange. Ryan claims garlic sea-stars get their name from their flavour. ("You'll see if you lick them.") He advises us to swim with our faces forward to better see fish against the horizon in these sunless shallows. He points out sea cucumbers, an octopus called Steve that often comes out to greet snorkelers, and two types of jellyfish small as contact lenses, scattered like see-through confetti among the kelp.

"There's also a big orange type of jellyfish the size of a basketball that you might see, but if you do then leave it alone, because it will sting you. But the only part of your body that's showing is your lips, so you'll only get stung if you kiss it."

Air in a back pocket of the dry suit keeps us buoyant at the surface. When I tire I flip on my back and bob up and down under a wide, cloud-filled sky. Small waves slosh, creating bubbles against the rocks that fizz on my face. It's a calming experience, almost dreamlike, rather than the unpleasant challenge I'd been expecting.

Back on the beach, we drag off our dry suits, taking particular care with our gloves. "My wedding ring flew right off once and I never found it," cautions Ryan. "So if you see a gold ring next time you're snorkelling, you'll know it's mine." We're all exhilarated that we've survived our plunge into near-polar waters and seen so many unexpected creatures. Bald eagles sit on trees overhead, impervious to our chatter as we sling our gear into Ryan's van.