His craft was going “full throttle” when Shepherd handed the wheel to an excited Miss Brown to give her “an extra thrill”.
But the speedboat hit a submerged tree trunk and capsized, the Old Bailey was told. Miss Brown was hurled into the freezing water and drowned.
Shepherd was found clutching the upturned boat and screaming for help.
Business consultant Miss Brown was killed near Wandsworth bridge, south-west London, in December 2015. Web designer Shepherd, of Abergavenny, Wales, denies manslaughter by gross negligence. He is being tried in his absence.
Questioned by police, Shepherd said he and Miss Brown had shared two bottles of wine at The Shard before taking champagne and going out on the boat, jurors heard.
He told the officers: “Neither of us were wearing life jackets, although there were two between the seats.
“She would not have known they were there and I did not point them out. I did not even ask if she could swim.”
Shepherd, who was living in a houseboat at Hammersmith, west London, at the time, had taken other women out on the boat, the court heard. In the months before Miss Brown died, he had twice been warned about speeding and failing to wear life jackets, it was alleged.
On the night of the tragedy, the boat was estimated to be doing an average 30 knots (34mph) on the way to Westminster and between 22 and 29 knots on the return journey.
The speedboat had several defects, including “poor and sloppy steering” and a “partially opaque” windscreen, the jury was told.
Frederick Psyk, a former boat owner, said he saw the craft “going flat out”. Riverside-based Lambeth firefighter Chris Leeming said he heard a high-revving engine and a woman’s voice whooping, “laughing like she was having a great time” before the accident.
The trial continues.