The boy, one of up to 10 child witnesses, was giving prosecution evidence in the trial of 11 men, including his father, and a woman accused of involvement in a paedophile gang in South Pembrokeshire, west Wales.
In a recorded video interview with a woman social worker and a detective shown at Swansea Crown Court, the boy was encouraged to draw the boat incident which he said happened about a mile offshore.
He said his father and two other men surrounded him in the boat while he was threatened.
The social worker asked: 'Why did he drop you in the water?'
The boy replied: 'To try and frighten me not to tell what happened to me.'
He also claimed his father forced him to carry out oral sex on him during the boat trip. 'My Dad used to make me do it to him,' he said.
The social worker asked: 'What were the other men doing when this happened?'
'Laughing,' he replied.
The boy said that during one trip his father cut him on his left wrist with a knife.
The social worker asked: 'Do you think they wanted to go on the boat because they wanted to be nice to you?' The boy replied: 'That's what I thought at the time.'
Earlier, the boy said he and other children were forced to go down into the inspection pit of an isolated garage where naked adults abused them.
He identified a number of locations in isolated parts of South Pembrokeshire and later took his social worker and a detective to them, the court heard.
His description of the boat incident came during more than three hours of video evidence played to the court.
The judge, Mr Justice Kay, and the 25 barristers removed their wigs to make the court seem less intimidating to child witnesses giving evidence through closed-circuit television.
The defendants have pleaded not guilty to a total of 42 charges including conspiring to commit serious sexual assaults, indecent assaults and acts of gross indecency against children.
Playing with a toy gun as he talked to the social worker, the boy said his father had once threatened him with a shotgun at an isolated barn where children were abused.
His father at first aimed the weapon at him but then pulled it back and fired. The shot hit the top of the barn.
'It made holes in the barn,' the boy said.
He also named one of the other defendants and said he beat him with his fists and tied his wrists with rope.
'He used to hit me, tie me up and throw me everywhere. He used to give me a black eye and he made me take my clothes off.'
The trial continues today.