Grimsey's win continues an outstanding year. In September, he shaved two minutes off the record for the English Channel, recording a time of 6h 55m.
He also won this year's FINA open water swimming grand prix circuit, is a three-time Australian open water champion, and the current New Zealand Open Water 10km champion.
In the women's race, New Plymouth's Charlotte Webby was even more dominant. She won by over two and a half minutes, ahead of 17-year-old Aucklander Kirsty Wannan.
But despite her win, Webby said the race could have gone better. "My goggles kept fogging up today, which wasn't ideal. But in a way I'm pleased these kind of issues came out today, so I can sort them out ahead of other five races in the series."
Both Webby and Grimsey have earned 100 series points for their wins today, as they chase the $20,000 grand prize for the series winner.
Grimsey's confirmed he's entered all of the other five rounds of the State New Zealand Ocean Swim Series, to try and get the cash.
The other rounds include Auckland's iconic State Harbour Crossing on December 2. The other four races are held in the New Year, in Wellington, Akaroa, Mount Maunganui and Takapuna.
The numbers entering the event were up on last year despite the race moving to an earlier date.
One swimmer, Russell's Margot Forrest, said most swimmers would have finished with slower times this year.
"I think there was a stronger current in the channel than last time and the winning times were slower, I felt like I was constantly having to redirect myself after finding myself heading south," she said.
She said the day couldn't have been better and the water was very calm but she had been surprised by the attention she got when she got out of the water.
Forrest, who lost her left leg in 2007, was mobbed by photographers as she came out of the water and wasn't sure of the time she posted.
There were other celebrities in and on the water with former Northland All Black Ian Jones completing with his son Flynn and Kerikeri Olympic Silver Medallist Blair Tuke on the water.