A boat owner charged after a fatal marine incident on the Brisbane River has claimed a "drug crime syndicate" may have been responsible.
Stan Anthoulas, 54, says he received death threats, was "followed by cars" and had been confronted by a "predatory group" before the February 2016 tragedy that claimed a woman's life.
"I believe it is a drug crime syndicate. I have evidence that ... would be very helpful to the police but ... they keep shutting the door on me," Anthoulas told the District Court on Monday.
Representing himself, Anthoulas said he had been in a relationship with the woman but she was involved in the "wrong crowd" and issued him a warning about a year before the incident.
"She told me to be careful, that my life could be in danger," he told the jury.
"From that moment on I was being followed by cars."
Anthoulas provided licence plate numbers to police but was told "there is nothing we can do".
He also believed there were other suspicious incidents in the lead-up to the boat sinking.
Anthoulas insinuated foul play after a wheel fell off his car while driving on a Brisbane highway before his trailer suffered the same fate days later.
"I have been driving for 30 years or so. I have never lost a wheel. Yet two in the space of a week?" Anthoulas told the jury.
Death threats then started before Anthoulas claimed he was forced to evade a "predatory group" waiting for him when he arrived to meet a friend at a Brisbane hotel.
When Judge Tony Moynihan asked what was the relevance of his claims, Anthoulas said: "I believe on that day of that boating incident ... there was a predatory group waiting - including police - and my boat was possibly swamped deliberately."
Anthoulas dismissed crown prosecutor Danny Boyle's claim that his boat had hull fractures, instead pointing the finger at police.
The boat was refloated for almost six hours of testing by water police who found Anthoulas' vessel could have been "dangerously unstable", Mr Boyle said.
"In the process of the retrieval of my boat it is quite possible that is when the damage occurred," Anthoulas said.
He became emotional when he spoke about the fatal marine incident.
Anthoulas' vessel sank 50m off Murarrie as it queued up at a boat ramp returning from a fishing trip.
He had been attempting to remove a fishing net from the boat's propeller when he heard the woman say "oh no".
"I turned around and a wave had come over the back of the boat with some force," Anthoulas said.
Fighting back tears, Anthoulas described how he had been thrown from the boat before trying to save the woman who was trapped underneath the forward deck.
Anthoulas has pleaded not guilty to contravening the general safety obligation of a ship owner, causing death or grievous bodily harm.
The trial continues.