Divers from the Cardiff British Sub-Aqua Club (BSAC) will take on the challenge on Sunday, May 3.
They will walk from Penarth Lifeboat Station to the RNLI Visitor Centre on Barry Island, via Barry Dock Lifeboat Station, while carrying a two-person kayak.
Steve Lloyd, chairman of Cardiff BSAC, said: “We wanted to organise a challenging fundraiser for the RNLI and show support to the volunteers from our local lifeboat stations at Penarth and Barry Dock.
“The crews are always there ready to answer the call for help and save lives at sea.
“This is going to be a tough challenge.
“Carrying a heavy kayak for ten miles is no small task as well as the hills to contend with along the way.
“If RNLI crews are prepared to launch into raging storms, we can certainly cope with a bit of rain.
“We’re determined to complete the challenge and raise as much money as possible for this vital cause.”
The kayak has been loaned by Cardiff Bay Kayaking, and two teams of six will take turns carrying it along the route, which passes through Penarth, Sully and Barry.
Penarth RNLI volunteer Sara Siddall said: “As a charity we rely entirely on public donations and the dedication of our volunteers to save lives at sea.
“We’re delighted that Cardiff BSAC have chosen to fundraise for us and support our lifesaving work.”
The team expect to reach Barry Island at around 2pm.
Avril Kitchen, head of fundraising for the Barry RNLI Visitor Centre, said: “We’d love people to come down and be part of the atmosphere, cheer the team over the finish line and show their support for an incredible challenge.”
