Illness forced Olympic gold medallists Alex Gregory and Peter Reed to pull out of GB Rowing's winter assessment camp in Lincolnshire on Saturday.
The duo, who won gold as part of the coxless fours at London 2012, had been due to race in the men's pairs, won by Andrew Triggs Hodge and Mohamed Sbihi.
George Nash, who won bronze at London 2012 with Will Satch, was second as he paired up with Rowan Lawson.
Olympic single scull bronze medallist Alan Campbell won the 5km time trial.
A competitive race in Boston also featured the in-form Charles Cousins, Australian Youth Olympic medallist Jack Beaumont and two-time Olympic medallist Matt Langridge, who made his season's debut after a long lay-off.
The equivalent women's event was won by Beth Rodford, a world quad gold medallist, ahead of Vicky Thornley.
Another Olympic medallist Richard Chambers, won the lightweight men's single cull from Adam Freeman-Pask, Jamie Kirkwood and his younger brother Peter, who also won Olympic silver in the men's lightweight fours.
The women's event was dominated by Imogen Walsh and second-placed Kathryn Twyman.
The five-discipline meeting was a chance for GB Rowing to assess the form of established and emerging British rowing talent.