Image copyright Reuters Image caption Neighbours accuse China of militarising its fishing fleet

If you want to understand the way China really feels about its controversial claim to huge swathes of the sea off its southern shore, then the island of Hainan is a good place to start.

This is a place where everything is bent towards justifying and upholding that assertion of sovereignty, from government and military policy, to fishing and tourism, and even history itself.

We came to the fishing port of Tanmen, on Hainan's east coast, because of recent state media reports about the existence of an extraordinary document - a 600-year-old book containing evidence of vital, national importance.

'Iron-clad proof'

The book, in the possession of a retired fisherman called Su Chengfen, is said to record the precise navigational instructions by which his long-distant forefathers could reach the scattered rocks and reefs of the far-flung Spratly islands, many hundreds of nautical miles away.

China's insistence that these features are Chinese territory rests largely on a "we were there first" argument. So 81-year-old Mr Su's book, "cherished" and "wrapped in layers of paper" is apparently a kind of maritime Holy Grail.

In fact, the reports suggest, it offers nothing less than "ironclad proof" of China's ownership of the South China Sea.

So we went to meet Mr Su and found him busily building a model boat in his front yard, a short walk from the beach.

"It was passed down from generation to generation," he tells me when I ask about the book. "From my grandfather's generation, to my father's generation, then to me."

"It mainly taught us how to go somewhere and come back, how to go to the Paracels and the Spratlys, and how to come back to Hainan Island."

But then, when I ask to see the document - the existence of which was, just a few weeks ago, being so widely reported in China and beyond - there's a surprising development.

Mr Su tells me it doesn't exist.

"Although the book was important, I threw it away because it was broken," he says.

"It was flipped through too many times. The salty seawater on the hands had corroded it... In the end it was no longer readable so I threw it away."

Image copyright Reuters Image caption China has been steadily reclaiming land around reefs in the South China Sea

Q&A: South China Sea dispute

China's Island Factory

Why are Chinese fishermen destroying coral reefs?

Beijing accuses US of militarisation

Whatever it was, Mr Su's book is not, it seems, any longer ironclad proof of anything. Except perhaps China's Communist Party-controlled media's willingness not to let a few facts get in the way of the official narrative.

We leave Mr Su's house, a little baffled by the experience, and are given another glimpse of Hainan's readiness to control the message when it comes to the South China Sea.

Everywhere we go, we're followed by a number of blacked-out government cars; from the port where we try to interview fishermen, to the fish market where we speak to traders, and all the way back to our hotel.

The attention seems a little unnecessary as almost no-one we approach wants to talk anyway.

And those that do, tell us nothing more controversial than a simple repetition of the official line, that the South China Sea belongs to China because Chinese fishermen were there first.

But the authorities are taking no chances. We learn soon afterwards that one of those who did agree to answer a few of our questions, a boat captain, was immediately picked up and questioned by the police.

Battle of propaganda

All of this comes, of course, amid the much-anticipated international court ruling on the South China Sea, expected some time in the next few weeks.

The Philippines has gone to the Permanent Court of Arbitration in The Hague to ask for a technical ruling about the extent of the territorial waters that can be claimed on the basis of the possession of various coastlines, islands and rocks.

Image copyright AFP Image caption China's claim over the islands is an inflammatory issue in the Philippines

The ruling is not widely expected to favour China, and may even go so far as to invalidate its most expansive claim - the "nine-dash line" that encompasses up to 90% of the disputed Sea.

China has, perhaps unsurprisingly, said it will neither take part in the tribunal nor accept the authority of its ruling.

Which is why it has instead been vigorously defending its position by other means; ratcheting up the propaganda - particularly its insistence that history is on its side - and engaging in a diplomatic push to win allies to its cause.

This may help to explain why a foreign journalist's presence in Hainan at this particular moment in time is likely to attract such close attention from the authorities.

Although in our case there may have been another reason: we were, perhaps, asking too many questions about Hainan's notorious "maritime militia".

China has been known to be giving its fishermen military training for decades.

Media playback is unsupported on your device Media caption The BBC's John Sudworth went to track down the famed 600-year-old book

But in recent years, the number of militiamen on fishing boats is reported to be increasing and their actions appear to be becoming more assertive in helping to underwrite and enforce China's sovereignty claims.

Their strategic advantage is that they can be, and often are, used for irregular military engagements - occupying territory at sea, carrying out surveillance or harassing other vessels - while operating under the guise of civilian fishing boats.

The activities of the militia units in the port of Tanmen have been well documented.

They even have their own headquarters inside the town's government compound, honoured with a visit in 2013 by the Chinese President Xi Jinping.

Despite our efforts though no-one would talk about the role this shadowy force plays within China's fishing fleet, and the more we ask, the more intense the tailing and government surveillance seems to become.

Image copyright Pentagon Image caption It is not only at sea that there is a danger of a miscalculation or accident

Prof Andrew S Erickson from the Chinese Maritime Studies Institute at the US Naval War College believes the presence of the militia in already troubled waters raises risks.

"I see a tremendous risk of miscalculation and escalation," he told me.

"The current approach that China is taking to the use of its maritime militia not only puts them in danger, [it] puts any other individuals and vessels around them in danger and it indeed imposes a risk of force being used against them by the US and other forces in legitimate self defence or to ensure the legitimate passage of vessels."

Image copyright Getty Images Image caption In 2013, during a standoff with Vietnam, China sent one of its largest ever recorded fishing fleets to disputed islands

A pyrrhic victory?

And that risk may rise even further, he suggests, after the Permanent Court of Arbitration Ruling.

"When the arbitral tribunal finally hands down some sort of a ruling I think China is going to try to find a way to concretely register its opposition, its resolve and its displeasure.

Media playback is unsupported on your device Media caption China's tours in disputed South China Sea

"And I think using maritime militia forces to further approach in close proximity and potentially harass US, Philippine and other vessels is something that policy makers from those countries must be prepared for."

So, while the Philippines could well soon be given a ruling that will vindicate its position, it may turn out to be something of a pyrrhic victory.

The international arbitration will not constrain China in regard to its expansive claims in the sea. It has already made that very clear.

But it may instead further convince the government and military leaders in Beijing that there is only one way forward - force.

Duty tourism

We end our trip to Hainan in the southern city of Sanya, watching a cruise ship set sail for the disputed Paracel Islands.

The five-day package tour began operating in 2013 and thousands of Chinese tourists have since taken the trip, which is not open to foreign passport-holders.

It's a bizarre holiday concept - a long voyage to take in a few reefs and largely uninhabited rocks, many miles out to sea.

Image copyright IHS Jane's Image caption China's extensive construction work on islands and reefs in the region continues to alarm and anger its neighbours

They are the same rocks, of course, that retired fisherman Su Chengfen's forefathers very likely did visit all those centuries ago.

There is certainly some evidence that complex navigational knowledge from ancient times has indeed been passed down, orally, from one generation to the next.

But the need to make all facts fit the official history appears to have magically turned Mr Su's heritage into hard, concrete evidence which is then published in national newspapers in the service of an argument which itself doesn't stand up to much interrogation.

Even if Mr Su could produce a 600-year-old book to show us, it would be proof only of the ancient use of the South China Sea, not necessarily ownership of it.

Many other South China Sea nations can, of course, also point to evidence that fishing communities along their coastlines have long been using the waters too.

But in China there is only one narrative and our experience in Hainan is a perfect illustration of how effectively that narrative is being defended and reinforced.

I ask one woman, as she prepares to board the cruise ship, why on earth she has chosen to spend her valuable vacation time visiting a few barren rocks.

"We're not going to enjoy ourselves," she replies.

"We've been educated since birth that it's our motherland's sacred territory. It's our duty to go and see."