Later, kayaks and stand-up paddleboarders held their own races over the course.
The fastest open-water swimmer was James Edmonds in just 20 minutes and 24 seconds. He was followed by Steve Ferguson in 20min 37sec and Natalie Steele in 20min 41sec.
The first swimmer to complete a mile was actually 11-year-old Jack Northmore, swimming in Tinside Lido.
Swimmers aged under 17 were not allowed to compete in open water.
The fastest finisher of all — also in the Lido — was 15-year-old Matthew Rose, who started after Jack and came home in just 18 minutes.
They were joined in the Lido by Charlie Hyland
, at seven the youngest swimmer in the event; Ella Windle, 9, and Danielle Taylor, 16.
, the Blue Mile operations director, said: "This part of the event highlights what a wonderful facility for the city is the Tinside Lido."
Three staff from Cotswold Outdoor's headquarters in Wiltshire swam in the first open-water wave.
Zoe Averill, 35, said: "It was really good and well organised. It's great to support something to do with the environment."
The trio were also swimming on behalf of Project Kaisei ('Ocean Planet' in Japanese), which is working to reduce the amount of plastic debris in the oceans.
Catherine Spear, 37, was the fastest of the three, completing the mile in 34 minutes and 54 seconds.
People came from across the world to complete a mile in, on or next to a 'water environment' to raise money for WWF-UK.
Barry Harding, head teacher at Leigham primary and a member of the Plymouth-Ghana Link, walked the Blue Mile along the Hoe with a group of teachers and pupils carrying buckets of water on their heads, African-style.
"This event is about the sea," Mr Harding said. "But we think fresh water is just as important.
"We want to raise awareness of the plight of millions of people who don't have the luxury of fresh water the way we do.
"People in Ghana have to carry water on their heads every day."
Nicola Martin, 10, said that carrying the water from the National Marine Aquarium to West Hoe was hard work.
"I understand what it's like for African women," she said. "I'd have a sore head every morning if I lived there."
Conrad Humphreys, the Plymouth-based round-the-world yachtsman and founder of the Blue Project, said: "This was, I hope, the beginning of a new opportunity to bring life back to the waterfront."
Yesterday the Blue Mile events centred on the Mount Batten Centre, where partners including the University of Plymouth supported low-cost taster sessions to encourage greater participation from disadvantaged groups across the city.