Kamil Zydek, 34, denies being part of a team of three men who planned the killing of Mr Stolarek, 41, and then the disposal of his body during the early hours of November 28, 2019.
The body of Mr Stolarek, a Polish national, was found by a service engineer at the Chain Bridge Road pumping station in Boston on January 12, 2020.
A jury heard police launched a major police investigation and two other men, Lukasz Ferenc and Adam Kaminski, have already been convicted of Mr Stolarek's murder.
Giving evidence at Lincoln Crown Court, Zydek broke down in tears as he admitted being in a car when Marcin Stolarek was put in the boot by Ferenc and Kaminski.
Zydek said Ferenc gave directions to the driver of the Vauxhall Astra, Artur Klosowski, after Mr Stolarek was taken from Kaminski's home in Union Court, Boston.
The prosecution allege Zydek then helped Ferenc to dispose of Mr Stolarek's body.
In his cross-examination of Zydek, prosecution barrister Gordon Aspden KC asked Zydek: "You went with them to get rid of a dead body?"
Mr Aspden continued: "It was you and Ferenc who then dumped a dead or dying body into the water?"
"I was still in the car," Zydek replied.
Zydek then admitted returning to his girlfriend's home in High Street, Boston, with Ferenc.
He denied a prosecution claim that a "bottle of whisky came out," telling Mr Aspden: "I don't know, I don't remember drinking."
The jury heard Ferenc visited Zydek in December 2019.
Zydek was asked why the man who he claimed "had done all these terrible things" was still in contact with him.
He replied: "I was carrying on with my life as normal as I believed Marcin could have survived."
Zydek confirmed to Mr Aspden that he had seen Marcin on the kitchen floor at Union Court, wrapped up in a blanket and placed in the boot, and then thrown in the water.
Mr Aspden asked Zydek: "You genuinely thought he was still alive?"
Zydek replied: "I thought Marcin would have been strong enough."
During his evidence in chief, Zydek told defence barrister Alisdair Williams KC that he asked for Mr Stolarek to be taken to hospital.
The trial continues.