It is the first world championship medal for the Cambridge-based 23-year-old, who has burst onto the elite stage after being unused as a reserve for the lightweight four at last year's Rio Olympics.
He finished nearly two lengths behind O'Donovan, with Norway's Kristoffer Brun third.
The number of remaining New Zealand finalists swelled by another three as Robbie Manson (single sculls) and the double sculls combinations of Olivia Loe and Brooke Donoghue, and John Storey and Chris Harris all achieved their top-three semi-final requirements.
Manson, the pre-regatta favourite, once again trailed home some key rivals. He was third behind behind Cuban Angel Fournier Rodriguez and four-time world champion Ondrej Synek of the Czech Republic.
Manson, who dominated at two World Cup regattas this year, was second in his heat and third in his quarter-final.
The 27-year-old is either preserving energy in the relentless Florida heat or is possibly paying the price for a build-up spoiled by a rib injury.
There were no problems for Loe and Donoghue, who also won twin golds at both World Cups entered this year.
They dominated their semi-final and were the fastest qualifiers, nearly a second quicker than the other winners, Meghan O'Leary and Ellen Tomek of the United States.
Storey and Harris were runners-up in what was easily the faster semi-final, finishing just under a second behind French winners Matthieu Androdias and Hugo Boucheron.
Single sculler Hannah Osborne's sixth placing in her semi-final means four of New Zealand's 13 boats failed to qualify for A-finals.
The three finalists chasing medals on Sunday (NZT) are women's pair Kerri Gowler and Grace Prendergast, men's pair James Hunter and Tom Murray and women's double scullers Jackie Kiddle and Zoe McBride.
