More than 20 divers at the Southsea branch of the British Sub Aqua Club walked the 6.9-mile distance from the Spinnaker Tower to Fort Widley on Saturday.
All proceeds will be going to Cancer Research UK, the Royal Lifeboat Institution and The Royal Marsden HospitaI in London, which was recently severely affected by a fire.
But this is not just another fund-raiser for the club.
Dawn Barnard, secretary of the branch, came up with the idea after losing two best friends, Emma and Gill, to cancer within eight months of each other in 2007.
She said: 'I wanted to do the walk for Emma as she wanted to raise money for the Marsden, so I did it for her. We have raised £2,000 so far and there are more donations to come.'
Neil Jeffery lost his wife to cancer in September 2007.
He said: 'I have lost my wife, who was only 45, with two young sons, and recently my father.
'We especially owe a great debt to The Royal Marsden.
'We were told there was nothing more they could do for her but the Marsden still treated her with a positive approach.'
Neil took along his two sons, Conor, aged 14, and James, 16, to the walk so they could pay tribute to their mother.
Conor said: 'It was very cold being in a wetsuit for so long but raising money for the charity was important to me.'
James added: 'When my dad told me about the walk I wanted to take part for my mum.
'I was thinking about her when I was doing it and it kept me going.'
Another member who had his wife close to his heart during the walk was Richard Hobson, 56. He said: 'I lost my wife to lung cancer in January 2007 and I think about her every day – the walk was my tribute to her.'
The group began the walk at 9am from the top of Spinnaker Tower and ended at their club at Fort Widley at 3pm, carrying collection buckets for people to donate en route.
For details on how to donate go to www.southseasubaqua.org.uk