What colour, I wonder, splashes across your mind when you hear the words, "Pacific Ocean"? And "Indian"? Is it some variation on blue and green? Now try the same with "Atlantic". Maybe it was boyhood exposure to Nicholas Monsarrat's The Cruel Sea that gave me my hard Atlantic grey. Not necessarily a drab colour, but after repeated readings of the convoy classic, those wolfish waters were definitely of steel and lead. Paradoxically, my previous experience of the ocean  summer holidays in Pembrokeshire and Cornwall  had given my Atlantic an undoubted turquoise tint. Monsarrat had done what the best authors manage to achieve: a change of colour scheme.

Embarking on Simon Winchester's new geographical biography  a topography?  I wondered if he too could possibly repoint my palette. The Pond, as it is dismissively known, is certainly in need of a revamp. Its waters have been despoiled and plundered to the brink of disaster  and in the case of the Grand Banks cod fishery, sent over that edge into deathly extinction. High above the ocean surface sits an upper veil of aeronautical pollutants, the result of more than 1,300 flights per day, then comes a lower shroud of marine exhaust (greenhouse gas production from shipping, Winchester tells us, is the global equal of aeroplanes). Below the waves things only get worse: radioactive pollutants dumped by Britain in one spot 400 miles west of Land's End amount to 29,000 tons.

This saltwater expanse that once inspired awe and reverence for its size  one fifth of the Earth's surface  as well as for its ferocity and fertility, now lies beneath our transatlantic flight paths, besmirched and benighted, an irritating interlude that is two films wide.

The Atlantic Ocean as we know it, Winchester points out, did not actually exist until human explorers, very slowly at first, teased the outline into being. The Phoenicians were the pioneers in the seventh century BC, scouting for the imperial purple dye found in the murex sea snails of the Moroccan coast. Later came Norsemen such as Leif Ericson, battling across from Greenland to establish the first European outpost in Newfoundland.

As for the subsequent achievements of mariners such as Cabot and Columbus, Winchester does not linger long, perhaps judging them too familiar. Instead we get fascinating excursions with men such as Gil Eannes, a canny Portuguese sailor who in 1434 pulled off a trick that had foiled men for centuries: the doubling of Cape Bojador in West Africa. With that problem solved, the way was clear for the Atlantic African coastline to be mapped and for others to discover routes westwards.

This then is a book about the people who travelled the Atlantic, a book of human geography. Do not expect great undersea excursions to view alien creatures and hear about their lives. If cod, whales and a few others get a mention it is purely as human plunder; there's none of the depth here of, say, Mark Kurlansky's Cod or Philip Hoare's Leviathan. And therein lies a hazard that Winchester struggles to navigate: the wrecking rocks of the Reader's Digest narrows, a dark and dangerous passage littered with the broken spines of literary works that oversimplified their cargo. All too often my mind was charmed away to a previous, far more alluring and exhaustive volume on a particular subject. His account of the slave trade, for example, set me diving towards Adam Hochschild's Bury the Chains. Captain Winchester did at that point rush forward with a slim anecdote of Cudjoe Lewis, the last slave to be transported across the Atlantic in 1859. Phew! I was still on deck, albeit clinging to the taffrail.

Where the book really starts to engage (hardly a surprise from an author who is an inveterate traveller) is on the subject of the glory days of trans-atlantic passenger traffic. Winchester was fortunate to catch the last hoot of that majestic period, taking a berth on the Empress of Britain in 1963, where he stumbled upon a lost world of civilised travel: "11am. Bouillon on the Boat Deck" announced his magnificently ornate ticket. Here his storytelling abilities shine, with personal anecdote and research expertly woven. He takes us through a giant fleet review, from the first post ships of about 1700, past the peak of the second half of the 19th century, when 30 million emigrants crossed, and beyond to the age of the great liners. Digressions on lesser known characters such as Malcom McLean, the North Carolina truck driver who created containerised shipping, do not distract. Unfortunately, in other sections of the book, there is plenty that does: overlong and uninformative lists, patches of bland description, and an ill-judged pairing of coastal cities that simply doesn't work.

I stayed on board, however, got accustomed to the leisurely roll and pitch of Winchester's prose, and eventually reached an Atlantic creature without any economic role: Prochlorococcus, a floating cynobacterium so tiny that it was not discovered until 1986, when it was realised that this was the most numerous species on the planet. It is also, Winchester relates, responsible for one fifth of the oxygen output, and appears to enjoy increased sea temperatures in times of global warming. Our saviour turns out to be a single-celled bacterium. Now that did shift my colour balance a little. The Atlantic is still largely grey, but with hopeful hints of azure blue creeping in.

Kevin Rushby's Paradise: A History of the Idea That Rules the World is published by Robinson.