Josh Almond was asked to take part by Jessica Sanderson, who works in KSA's restaurant, after she learnt that he was a keen swimmer.
The pair were part of a four-person team representing Jessica's Tang Soo Do martial arts group.
The group had to swim 5k between them (200 lengths) and while Josh had originally planned to swim 20 lengths, he managed 40 on the day.
Josh, who is a member of the water polo club at Kettering Swimming Pool and swims twice a week, said: "It's the most I have swam in one go. I felt better than I thought I would - I didn't think I'd be able to swim 40 lengths in that time. I've raised about £100 so far."
It was the fourth Swimathon that Jessica has taken part in and she swam 60 lengths in 34 minutes.
Year 7 student Jodie Lovell also took part in the Swimathon but with another group.