An 82-year-old retiree and a 12-year-old schoolboy are among 1300 people set to swim the Auckland Harbour Crossing today.

Both John Marshall and Alexey Blinov - the oldest and youngest competitors in this year's event - say the sense of accomplishment they get from completing a race is what motivated them to enter.

Year 7 Remuera Intermediate pupil Alexey is a bit nervous about swimming the 2.9km from Bayswater on the North Shore to Viaduct Harbour.

"It's probably going to be very tiring. It's probably going to be quite cold," the 12-year-old told the Weekend Herald.

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Although he had been swimming at pools and beaches since he was 6, today's Harbour Crossing would be Alexey's first open water swim.

"I'm nervous and excited just because of the currents," he said.

Alexey's 14-year-old brother is also competing in the event and their parents had been supportive of the goal, Alexey said.

He wasn't too bothered about his time and instead would focus on each stroke, making sure he was doing the best technique possible.

"Just right now I'm thinking about if I can complete it. That's my goal right now."

He said hearing that an 82-year-old was also doing the Harbour Crossing was "really great".

John Marshall, 82, took up swimming about five years ago. Photo / John Borren
John Marshall, 82, took up swimming about five years ago. Photo / John Borren

"He's just going for what he wants to do. That's awesome."

It will be Bay of Plenty man Marshall's fifth time swimming the Harbour Crossing and although he usually thinks "what the hell am I doing" when he's in the water among flailing arms and legs, the challenge of it keeps him coming back.

"It's good when you finish it. Even the training is a challenge and an achievement," he said.

Despite never having swimming lessons as a child, Marshall took up the sport to keep fit about five years ago after retiring from a 50-year career as a lawyer.

"When I decided to train for these ocean events [and] I got back in the water, I was able to swim two lengths of the pool without stopping, had a breath. Then I gradually increased that," he told the Weekend Herald.

"By the time I got to 10 lengths on end without stopping, then the rest was reasonably easy. When I got to 120 [lengths] I bought a wetsuit and I suppose the rest is history."

He is also competing in other ocean swim events this summer and trains by swimming about 3km in a pool near his home in Papamoa about five times a week.

But swimming in the open ocean comes with risks that are hard to prepare for.

"I can only breathe from one side and I'm always concerned that if the water is rough from that side that I'm breathing from I could well be history so I always have that concern. But so far I've been able to cope," Marshall said.

His family is yet to raise any concerns about the pensioner's ocean-going pursuits though.

"They probably think 'this stupid old bugger'."

The Auckland Harbour Crossing starts at 7.15am at Quinton Park in Bayswater. The race is part of the Banana Boat Ocean Swim Series that runs from now until April at beaches around the North Island and in Nelson.