The world has been talking about Seaspiracy. This is a film which claims that thanks to the greed-crazed behaviour of fishing boats and their owners the oceans will be empty of fish by 2048. The thing ends with a plea to viewers from the narrator, Ali Tabrizi, a half-shaved snowflake in a beanie with a powerful sense of his own intrepidity, to renounce once and for all the eating of fish, by which means the oceans will be saved.

The film has been extremely popular, and makes some interesting and alarming points, some of which are true, others not.

The learned article on which it bases its contention that the seas will be empty of fish by 2048 did not claim that the seas would be empty, only that fish populations would be reduced by 90%, bad enough to be sure, but a level from which recovery is possible.

The film’s claim that forty per cent of plastic pollution comes from fishing gear rather than land-derived plastic dates from the 1980s, and is specific to the Pacific Garbage Patch; the current more-or-less-global figure, alarming enough in all conscience, is 20%.

And the constant dissing of conservation organizations is both wearisome and inaccurate – you can prove anything by crafty editing, but that doesn’t make it true.

Still. Seaspiracy may be largely pap, but anyone who has been sailing since infancy will have noticed that the marine biosphere is increasingly in trouble.

This is in large measure due to the rape of the oceans by fishing fleets outside 200-mile limits, where just about anything goes.

Big fishing boats used to have owners; now they are owned by companies that are owned by other companies, and crewed by staffing agencies some of whom could be mistaken for old-fashioned slave traders, and flagged by states with only the vaguest connection to shipping (Problem with a Moldova-flagged ship, sir? You can always write to the President. Good luck with that, sir).

As for what they catch, much of it is unmonitored, vast quantities of it are wasted, and the less of it there is the more money it attracts, a fact that has not escaped the notice of organized crime.

Closer to home, marine conservation zones are open to most forms of destructive activity except waterskiing.

The great blue wonder that covers seven-tenths of the earth’s surface, on which the health of the land depends, is none too clean and increasingly depopulated, if not to the extent Seaspiracy claims.

The world is not merely a thing humans live on, but a vast and intricate system of which humans are only a part.

So what are we doing about it as leisure seafarers?

Greenwash prevails. Hilariously, enlightened marinas have linked their gyms – gyms? – to the electricity supply.

Various cruise ships, famous for pumping effluent into the sea through magic pipes and burning filthy bunker crude whenever possible, have made great play of banning plastic drinking straws from their bars, though they are silent on the composition of the little umbrellas.

Boatyards hammer up posters from the Green Blue insisting on the harvesting of every stray drip of antifouling from your Drascombe, while in the drydock round the headland supertankers are receiving multiple-football-pitch-sized coatings of toxic paint.

Rope manufacturers will swap your old rope for new recycled rope, though not if you are a fisherman.

Inshore fishing boats, in Devon as in Mauritania, scraping by with creels and nets and other non-destructive fixed gear, are inspected half to death by the government, which is witholding their subsidies, while the big government money goes to the gangsters devastating whole ecosystems with their pulse fishing and beam trawls.

Yachties can, alas, be part of the problem. Some appear to see efforts to protect the seagrass beds of Studland as fascistic repression. Their scepticism is assisted by the authorities, who, pausing only to tell the yachties to suck it up, nip out to lunch with the trawler owners’ agents and give them permission to go beam-trawling in the Dogger Bank MCZ and scallop-dredging in the Cardigan Bay MCZ – activities about as sensible as bulldozing cathedrals for hardcore.

Prevention is left to Greenpeace and its splendid boulders, and further out at sea the intrepid ecopirates of Sea Shepherd.

The actions of these organizations are about all we have got at the moment.

Meanwhile it looks to a cynic like me as if a slick of greenwash shines iridescent on the oceans; and rabblerousers like Seaspiracy, sensational but sloppily easy to refute, are not so much part of the solution as part of the problem.