Wealthy Christchurch businessman Alasdair Cassels and convicted fraudster Mike Swann have been accused by family members of being involved in the disappearance of a missing Marlborough Sounds charter skipper.

Accusations of murder or foul play were alleged by the children of William Kerry Blair during a coronial inquest into his March 2014 disappearance.

However coroner Marcus Elliott, along with police who investigated Blair's disappearance, found no evidence to support the family's suspicions.

Both Cassels, 67, and Swann, 56, have vehemently denied the allegations.

Reporting of the Blair family's allegations has been prohibited by suppression orders that have only now lapsed after a legal challenge by NZME.

Four scenarios to explain the 55-year-old's mysterious case were explored during the inquest: suicide, medical mishap or accident, foul play, and faked death.

In his findings, Coroner Elliott all but ruled out foul play or a staged disappearance, concluding it was possible that Blair either went overboard or that he took his own life.

"Although he died on that journey, the immediate cause and circumstances of his death remain unknown," the coroner found.

Blair was last seen at Cassels' idyllic, remote 169-hectare Erie Bay property in the Marlborough Sounds - where he worked for Cassels for eight years as engineer and skipper on his 27m luxury motor yacht, the Galerna - on Saturday March 8, 2014.

The 9m aluminium Senator boat, also owned by Cassels and named Erie, which Blair had set off in for "fish n chips" that day, was found drifting 200km off the coast of Taranaki nine days later. Blair was not on board and his body has never been found.

During the inquest, both Cassels and Swann gave evidence, which the coroner said was helpful to his inquiry.

Allister Davis, counsel for the Blair family, put allegations to them both that they were involved in Blair's death.

Davis asked Cassels: "Did anyone you know, or have you paid anyone to have a part in Mr Blair's disappearance?"

"No," replied Cassels, who was represented at the inquest by Richard Raymond QC.

"You know clearly a number of unsavoury people, as you've quoted knowing murderers and rapists?" Davis put to Cassels, who replied, "Yes, so we both do Mr Davis."

Blair's daughter Rochelle Foster told the coroner she felt the chance that Swann, Cassels, and a third person, the Erie Bay cottage caretaker Jacqueline Elizabeth Coplestone, could have been involved, was a "huge possibility that was never investigated" by police.

Police deny many of the family's claims.

They say they approached the "enormous" missing persons operation with a practical clarity that emotional and distraught family members could not.

Police say there were no indications of foul play for them to investigate at the time, though they "kept an open mind".

It wasn't unusual for people to disappear at sea, Cassels told the inquest.

"I have had two people crewing for me on Galerna that have been taken by waves, not from Galerna but from other persons after they crewed for me on that boat. I mean, seamen do go missing on boats," he said.

Later, he added: "I've had one person taken by a wave and one person, um, went missing pulling an anchor up."

A skipper who used to be employed by Cassels at Erie Bay, disappeared at sea two years before Blair vanished.

Experienced sailor Richard Rusbatch's unmanned yacht Honfleur was found with the engine still running, doing circles in the Bay of Plenty, north of Tauranga, in February 2012. The yacht's stereo was blasting at high volume. His body was never found either.

Coroner Wallace Bain found it was likely that Rusbatch fell overboard and drowned.

"There is nothing to indicate anything had occurred on board the boat by way of injury," Bain said.

Cassels, who is behind The Tannery boutique Victorian Arcade in Christchurch's Woolston industrial area, previously employed Scott Watson at the Erie Bay property to work on his boats.

He sacked Watson days before the disappearance of Ben Smart, 21, and Olivia Hope, 17, who were last seen in Queen Charlotte Sound on New Year's Day 1998.

Watson was found guilty of their murder in 1999.

During the high-profile double-murder trial, Cassels gave evidence to say that Watson had phoned him after police had seized Watson's yacht, Blade.

Cassels told the court that Watson had asked him whether police had dug up anything at Erie Bay. Cassels said he didn't know how to interpret the phone call. He also said he'd sold Watson an anchor, anchor winch, and a chain.

Cassels had also employed a man – who can't be named because of legal reasons - as a caretaker on the Erie Bay property where the worker grew cannabis. The employee also gave crucial evidence during the Watson trial and died in a 2003 drink-drive car crash while not wearing a seatbelt.

Cassels and Swann are friends going back "20-odd years".

Swann spent four years and eight months in jail for his part in defrauding the former Otago District Health Board of $16.9 million. He is subject to a $6m pecuniary order.

Nicknamed 'Mr Moneybags', Swann spent almost $11.6m on properties, boats, and flash cars, including a Rolls Royce.

Millions of dollars more remain unaccounted for.

After his release from prison on parole in 2013, Cassels put Swann up in his Governors Bay mansion, overlooking Lyttelton Harbour. Later, Swann was living in a house bus at a heritage-listed century-old home also owned by Cassels at the hillside Clifton suburb above Sumner. It was destroyed two years ago in a fire that police say was an arson attack.

Blair had been in a sexual relationship with Coplestone while they both lived at Erie Bay, the inquest heard, but it had ended well before his disappearance, with Coplestone saying he'd become "demanding".

Around 4-5 months after he disappeared, Coplestone – who was the last person to see Blair at Erie Bay – and Swann got together, they told the inquest.

"Yes, he's my boyfriend," said Coplestone, who also strongly denied allegations of foul play, which was supported by the police and coronial findings. "You don't get many blokes out in Erie Bay so when you do you make the most of them."

When the Blairs' lawyer asked Swann during the inquest if he had "rubbed shoulders with a lot of unsavoury characters" in prison, he accepted that he had. He admitted that he still wrote to inmates he'd met behind bars.

Pressed on whether he'd had anything to do with Blair's disappearance, Swann, now a project manager, said: "I was not involved in the disappearance of Mr Blair."

Davis suggested that if Blair had overheard Swann talking about "the missing millions", then he would have had a motive to disappear, which Swann rejected.

While Cassels said it was possible Blair was lost at sea, he agreed with the police conclusion that Blair had taken his own life.

Cassels, who provided the inquest with charts he'd composed from cellphone pings to plot Blair's possible route in the Senator, told the coroner his theory: "Well the theory is, I mean it's not a theory, I know in my heart what he did, and he went out, you know, probably sometime later on that afternoon on the Sunday he threw himself in the water, probably aiding his demise with my shotgun that went missing as well."

The inquest explored the possibility that Blair had taken his life in roughly the same area he lost mates when the fishing vessel Iona sank in 1993. Blair had helped in the search 21 years earlier and Coroner Elliott noted "it must have been a source of sadness for him, involving as it did the loss of fellow seamen".

Cassels also believed that the Iona could have played a part in Blair's thinking.

A 12-gauge pump action shotgun went missing from Cassels' Erie Bay property, which has a large house, tennis court, swimming pool, small vineyard, jetty and walking track, around the time Blair vanished. It's never been found.

The Galerna, originally built for the Danish royal family in 1973, was for sale at the time Blair went missing.

Blair was anxious about his future if the boat was sold, the inquest heard.

Cassels had promised him a severance payout of "about $10,000" and proposed a deal to work on the Galerna with the new owners for a three-month handover period.

A month or two earlier, Blair had given his boss "a rev up" over the uncertainty of the sale, which Cassels felt arose due to his employee's concerns about his future.

"By that stage, I was a bit… I was a bit sick of him actually," Cassels said.

"I mean, he was really a fairly useless article, and just not really performing his job."

Four years since his father's disappearance, son Dylan says he still doesn't know what happened.

He says he recently noticed a message sent via Facebook Messenger from his father the night before he disappeared. It said: "Report anything suspicious to police".

At the time, Dylan was editing a community patrol website and thought nothing of it.

Now, he wonders.

The 32-year-old labourer - whose suppression around his convictions for fraud has also been lifted - doesn't accept the suicide theory and he ruled out accident or medical mishap during the inquest.

He fears foul play and believes that "someone knows more".

Despite the police and coronial findings, Dylan has vowed to continue looking for answers.

"It is hard, and it does take its toll. I suppose sometimes it's just easier to just go along with the flow and just forget about it but that's not going to bring closure, that's not going to help the situation," he told the Herald.

"If Dad is still alive, I'd like to find out. You just simply don't know because there's no evidence to point to anything. You could put 10 different options in a hat, pull one out, and it's no less likely than any of the others."

Cassels said he was "amazed" by the family's accusations and again denied having anything to do with Blair's disappearance.

"I obviously didn't," he told the Herald.

"I just can't work it out. How could I have done something to [Dylan's] father? How could I have done that? I was asleep in my bed in Christchurch and [Kerry Blair] was sailing around Farewell Spit, how could I have done something? What bit did I do? It's just not possible."