Royal Navy personnel seized by Iran were blindfolded, bound and held in isolation during their 13 days in captivity, the crew have said.
The crew spoke for the first time about being held captive
They were lined up while weapons were cocked, making them "fear the worst", one of the 15 freed sailors revealed.
The crew were told that if they did not admit they were in Iranian waters when captured that they faced seven years in prison, a press conference heard.
Opposing their captors was "not an option," they said.
And after the 15 marines and sailors were seized they were subjected to random interrogation and rough handling, and faced constant psychological pressure, they said.
The navy has already begun a review of the circumstances surrounding the incident.
Royal Marine Captain Chris Air, 25, from Altrincham, Cheshire, said they had seconds to make a decision when confronted by the Iranians while carrying out a routine operation.
"We are aware that many people have questioned why we allowed ourselves to be taken in the first place and why we allowed ourselves to be shown by the Iranian authorities on television.
"Let me be absolutely clear, from the outset it was very apparent that fighting back was simply not an option".
"Had we chosen to do so then many of us would not be standing here today. Of that I have no doubts".
Had they resisted there would have been a fight they could not have won, he said.
"Fighting back would have caused a major international incident and an escalation of tension within the region," he said.
Two of the crew read out a prepared statement to the press conference at the Royal Marines Barracks at Chivenor, in north Devon.
Lieutenant Felix Carman, 26, of Swansea, south Wales, said the sailors and marines were on an operation on 23 March, 1.7 nautical miles from Iranian waters, when they were captured.
Cpt Chris Air said the crew had made it clear they were on a "routine operation allowed under a UN mandate" but the Iranians had a "planned intent."
"Some of the Iranian sailors were becoming deliberately aggressive and unstable."
Their boat was surrounded by six boats and rammed and they were trained with heavy machine guns and weapons.
The officer in charge Lt Carman said they were taken to a prison in Tehran where they were stripped and dressed in pyjamas.
They were kept in stone cells, sleeping on blankets and held in isolation until the last few nights and frequently interrogated.
Lt Carman said they were given two choices.
"If we admitted we had strayed, we would be on a plane back to the UK soon. If we didn't we faced up to seven years in prison".
The only woman in the group, Leading Seaman Faye Turney, believed for at least four days that she was the only one still being held.
"Like all of us she has been exploited," Cpt Air said.
Royal Marine Joe Tindell told how they feared for their lives in prison.
"We had a blindfold and plastic cuffs, hands behind our backs, heads against the wall. Basically there were weapons cocking. Someone, I'm not sure who, someone said, I quote 'lads, lads I think we're going to get executed'."
"After that comment someone was sick and as far as I was concerned he had just had his throat cut."
Lt Carman said they were only allowed to gather for a few hours together, in the full glare of Iranian media.
He said they only learned they were going to be released when they watched the Iranian president on TV.
"There was a huge moment of elation," he said.
Iranian TV has said it expected some of the British sailors would come under pressure from the UK government when they returned home to change their story.
The navy's review will look at the the circumstances surrounding the incident and the wider rules of engagement for UK forces operating in the area.