In the blood of every Briton runs at least a little seawater. We sing of the sea, romanticise our maritime heritage and regard the beach holiday as a nationally affirming birthright. Every year we potter in our millions down to the sea with bucket, spade, snorkel, jet-ski, paperback, shark defence kit and inadequate quantities of suncream.

Oh, we do like to be beside the seaside; but we have a strange way of showing it. For the past 300 years or so, we have poisoned and plundered the sea; we have destroyed the seabed, killed the fish and bemired the vast oceans with our waste.

We wring our hands at the pollution and devastation we have visited on the land, but because we cannot see what is happening beneath the dark waters surrounding this island we somehow assume it will mend itself.

Every so often a tar-soaked seagull pricks our marine conscience, but then the waves close over our fears again; the waves, it must be said, look much the same as they did in your youth, but they are not.

On every 100m of British coast there are, on average, 117 separate pieces of plastic. The North Sea cod, most notoriously, has been fished to extinction. Pockets of sea are dying, deprived of oxygen, suffocated by algae. Each British bottom trawler ploughs up some 13 square miles of seabed on an average fortnightly fishing trip. This, warns Richard Girling in his terrifying new book, Sea Change, is a way to earn a living that is approximately equivalent to “harvesting rabbits by bombing the Chilterns”.

Up to half of Britain’s entire biodiversity, some 44,000 species, exists in the fertile, shallow sea around Britain. This year the WWF estimated that of 16 key marine species and habitats all but two are in decline in UK waters.

While other countries have moved to protect their seas, for the past 25 years the waters around Britain have been systematically neglected and mismanaged. Australia has declared one third of the Great Barrier Reef a “no take zone”; New Zealand has 28 protected sea zones, while America, often an environmental laggard, has established numerous protected areas in either ocean. There are more than 500 marine preserves in the Philippines alone.

In British waters, by contrast, less than 1 per cent of the sea is afforded any protection whatever, and in much of that commercial fishing continues unabated. Latvia, with barely 300 miles of coast, has more marine reservations than Britain, with its 7,800-mile coastline. Lundy Island, off the Devon coast, was designated a marine nature reserve in 2003, one of just three small zones with limited protection from intensive fishing.

Lundy, however, is home to a small miracle. Around the island, after just five years of protection from trawling, the sea has started to recover: fish stocks have increased steadily, the variety of species is expanding within the safe zone, while lobsters around Lundy are said to be up to seven times larger, on average, that those in the neighbouring unprotected areas. In this tiny aquatic Utopia, the sea is coming back to life.

For most marine scientists, the implications are clear. The deterioration of the sea can be slowed if not halted by setting aside conservation areas. Quite how much of the sea would need to be preserved for sustainability is a matter of debate – some scientists argue that for every intensive fishing zone an equal area should be designated a “no take zone” – but the principle of defined marine parks is one that even the British Government, after years of neglect, appears to have hauled on board.

Earlier this year, Defra published a White Paper that laid out, for the first time, a comprehensive plan for protecting Britain’s marine environment and managing the entire delicate ecosystem. The marine Bill is the most sensible addition to British maritime life since the invention of the lighthouse: it envisages a series of protected national parks, perhaps rising to as many as 90 in all, including such sensitive areas as Dogger Bank and Scotland’s fragile coral reefs.

The Bill is not simply some worthy Magna Carta for beleaguered British fish, since it also sets out clear rules for exploiting the sea by fishermen, oil prospectors, dredgers and energy farmers. The Bill will make it far easier to build and operate offshore wind farms, developments to harness wave power, and schemes for storing carbon emissions from power stations in former oilfields. So far from ducking the issue, as successive governments have done, the marine Bill aims to balance competing interests and face up to the inevitable but not insoluble conflict between exploitation and preservation.

But in politics, as at sea, the weather changes quickly. The marine Bill, promised in Labour’s manifesto of 2005, was expected to become law within a year, but suddenly it seems to have slipped off the political agenda. Gordon Brown did not even mention marine protection in his summer statement, and the marine Bill is not included in his planned legislative programme for next year. The Bill has been kicked into the long seaweed. It is the big one that got away, again.

And so the steady despoliation of our seas – overfished, hopelessly underdefended, beloved in theory but abused in practice, a shared resource that is gradually being wrecked for all – continues. Only by protecting our marine habitats can we hope to preserve the livelihoods that depend on them – and our endangered taste for fish and chips.

We look back with nostalgia and pride on a time when Britannia ruled the waves. Britain’s failure, in modern times, to set rules for what goes on beneath its waves should be a source only of shame.

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