Antarctic Peninsula

Diving is a side programme on an expedition ship rather than the reason it sails. A small dive group, usually a dozen or fewer, shares the vessel with passengers doing landings, and the rhythm is one dive a day, occasionally two, slotted in wherever the ship's itinerary leaves room. The guide picks the site on the morning, after a long look from the bridge, so nothing is fixed in advance and a promised spot can quietly disappear overnight. Kit generally lives in a wet room and goes into the Zodiac already assembled.

Sites fall into a few recognisable types. Ice dives work the submerged face of a grounded berg, chosen aground precisely because a floating one can roll, and the interest there is blue-white sculpture rather than life. Rock walls dropping straight from shore are the opposite: dark, densely settled, worth a whole dive with the torch held close. Cobble and boulder slopes below the colonies are where encounters actually happen. Deception Island is different again, entered through the gap at Neptune's Bellows, with volcanic sand and sparse life inside the caldera.

Everything is engineered around cold. Regulators are cold-water rated and divers carry redundancy because free-flow, rather than gas planning, is the characteristic failure here; dry gloves are standard; and dives are kept short, often twenty to thirty minutes, with hands rather than gas or depth calling the end. The compensation is a seabed full of oddities, sea spiders and brittle stars and long nemertean proboscis worms, salps drifting through mid-water, and the specific business of surfacing into a bay of ice with the ship at anchor behind you.

It suits experienced drysuit divers who accept they are buying an expedition with diving inside it, and who will shrug when ice or weather takes a day away. November is the early end, with more snow and fast ice and fewer ships about; December and January bring the longest daylight and the colonies at full noise; February and March mean more open water, easier access to sites further south, and whales in the picture. Anyone counting dives should book elsewhere, since a voyage often yields fewer than a dozen.