A team of 64 students and four staff swam a total of 6,809 lengths at Littlehampton Wave.
The event raised a total of £1,453 for the charities, which both work on projects for cleaner water.
The senior student leadership team organised and promoted the event, supported by head teacher Chris Woodcock.
Ellie Lossing-Smith, year-11 pupil and student team member said: “We chose water awareness as our theme as it’s a very hot topic at the moment and it fits in with the school’s initiative of becoming as environmentally friendly as possible.”
Ava Hodgson added: “Part of water awareness is highlighting the issue of plastic pollution and trying to deter our classmates from using single use plastics.”
The members of the student team have been heavily inspired by 16-year-old climate activist, Greta Thunberg.
Ruby Childe, head student and student team member, said: “It is so important that we, as a school, do our bit to help.
“It’s amazing to see our generation campaigning for a better future and we hope our event inspires other schools.”
Student team member Lila Hite said: “As a school we are heading towards being more environmentally friendly and recycling as much as we can - which is why it was so important for us to raise awareness of a topic that fits in with that ethos.”
The students admitted that at first they were worried making sure they got the message across to the school and the community.
Ava said: “Members of the student leadership team, including myself, held assemblies for the school, sent a letter home to parents and went to speak to individual classes.”
Ellie said: “Organising the event wasn’t stressful, it was actually really enjoyable, we just had to decide what was going to work and be realistic.
“I felt a sense of reward – it was amazing to see how much we raised and how much fun everyone had.”
The students said they would love to do the event again.
Head student Eddie Pullen said: “The event was amazing this time round and hopefully we can make it even better next time.”
Year-seven pupil Holly Eastment, a swimmer at the event, said: “I had lots of fun swimming, it was tiring but fun.”
Fellow swimmer Eddie Cooper added: “I had lots of fun, too.”
Mr Woodcock was full of praise for the student team.
He said: “The event was amazing, the leadership team have been fantastic – their drive and determination is incredible, they are a credit to Durrington High School.
“We have lots of initiatives coming up to make us a more environmentally friendly school and they will be at the forefront of it all.”
The school chose to raise money for Oxfam and The Marine Conservation Society as both work towards ensuring clean water around the globe.