It's a sponsored swim - but with a difference, it's not all humans taking part.
Press Packer Katie, 12, from Suffolk, went along to a special dog water centre where owners and their pets were swimming lengths for 12 hours.
Her mum Jo explained that the hydrotherapy pools are used for dogs that have had surgery or need to do exercises in the water to keep healthy.
The 12 participants have raised £7,500 to buy a water treadmill.
Katie tells us all about it.