Richardson is a twice-Oscar nominated star known for her role as Rita Skeeter in three of the Harry Potter films – as well as Damage, Sleepy Hollow and Made in Dagenham, and TV’s Blackadder.
The 67-year-old WWF ambassador said: “I wanted to take on this swim with my willing chum Maggie as a challenge for both mind and body. But it’s also about raising awareness of the beauty and fragility of our natural habitats.
“We’re swimming in an area of Norfolk where saltmarsh, freshwater and sea all meet – an extraordinary habitat that shows just how connected everything is and reminds us how vital it is to protect and restore these links across our landscapes.”
The pair have been performing together in the Amazon Prime series Good Omens, starring David Tennant and Martin Sheen.
They are keen to raise the profile of WWF’s A Prescription for Nature campaign, that encourages people to get their daily dose of nature for personal wellbeing.