The campaign aims to drive participation and reinforce the event's inclusiveness and sociability.
23red has created on and offline executions including: A3 posters, appearing in leisure centres, schools, youth centres and sporting locations; promotional materials, such as t-shirts and window stickers; leaflets; banners; and a dedicated website.
The creative focuses on the idea of shoals with artwork featuring a shoal of swimmers in a pool, photographed from underneath.
They are all swimming in the same direction to reflect the inclusive nature of the event, where people come together to raise funds in much the same way that fish swim in a shoal to form a stronger unit.
Text on the poster reads "Everyone in the pool for the world's biggest swim" to emphasise the aim to appeal to people of all ages and abilities. It also includes the strapline "Let's all swim together for Marie Curie Cancer Care".
The image of the swimmers, created using a combination of stock shot photography and photo retouching, runs across all campaign materials.
Lesley Bray, marketing manager at Limelight Sports, the agency which manages the event on behalf of the Swimathon Foundation said: "23red did a great job crafting their creative vision into a finished piece of creative.
"They managed to create a stunning underwater shot that has become the creative focus for the entire event."
Sean Kinmont, creative director at 23red, said: "Developing an iconic image for Swimathon was a challenge we relished. In production terms, it really gave us a chance to demonstrate that even complex concepts can be turned into spectacular images."