Bexhill Seagals organised the event, inviting the public to join in a Mexican wave.
Despite the rain, organisers said more than 300 people joined in the protest.
In August, Bexhill and Normans Bay beaches were closed for two days after sewage was discharged into the sea, with Southern Water blaming a failure at its pumping station at Galley Hill for the incident.
Rachel Hills, organiser of Sunday’s event, said: “Residents have been expressing their despair at the ongoing pollution of our sea water. We have been advised against water and beach activities on many days during this glorious summer. The cost to the town in reducing visitor numbers, and damage to our reputation has been significant.
There were two Mexican waves held, one by the De La Warr Pavilion and a smaller one at the Richmond Road end of the promenade.
Rachel said: “People enthusiastically signed the petition demanding that Southern Water come up with a plan by Easter 2023 as to what they are going to do to stop polluting our water and people were pleased to take flyers so that they can sign the online petition at www.change.org/p/stop-polluting-bexhill-s-sea.
“Residents were not slow to express their outrage at the continuing damage to our marine environment, our physical and mental wellbeing and the negative effect on our tourism trade and reputation.
“On Sunday morning Southern Water was advising us not to swim and yet there were no signs on the beaches. So visitors turning up were unaware of the polluted bathing water, which made a Saturday bather quite ill.
"Strandliners, who regularly clean and audit the rubbish on our beach, have recent evidence of sanitary deposits on one of our beaches. Surely, our beach flag poles could be used to indicate the state of our bathing water – green for it is fine to swim and red you are advised to keep out of the water?
“The online petition has now reached 700 and will remain live for at least another 10 days and volunteers will be out with paper petitions.”