The sailing club car park and adjacent public beach have been closed as the beach is being used to store construction materials.
Joss Loader, independent councillor for Marine ward, said both should have been opened in the spring and the loss of parking spaces has put strain on the area over the busy summer months.
She said the contractor, Team Van Oord, is unable to commence work on the river bed until Southern Water points out the location of the sewer beneath.
“Southern Water needs to crack on and resolve the situation swiftly as this has dragged on for far too long,” she said.
“People are understandably asking how, with modern ground investigation techniques, can they fail to locate a sewer.”
Tim Loughton, MP for East Worthing and Shoreham, said he had written to Southern Water asking for an explanation.
The planned works are part of a £32 million Environment Agency scheme to renovate flood defences on both sides of the River Adur around Shoreham.
A spokesman for the Environment Agency confirmed it had been unable to locate the sewer mains and had requested the help of Southern Water.
The spokesman said the agency was making arrangements to remove the construction materials in order to reopen the car park, but the size and weight made the process ‘not simple’.
Southern Water was approached for comment.
Mrs Loader said she was frustrated at the inactivity in the area, which she said has not seen any work for weeks.
She said residents and visitors were having to park ‘all over the road’, making it difficult for buses to get through in Harbour Way.
“Residents fully support the tidal walls project but it has been difficult,” she said.
“The lack of communication is a recurring theme throughout this project, and a lack of timescales.”
The car park remains closed in preparation for work to begin, but Mrs Loader said there was no reason not to temporarily open it and then close it again when the work is ready to commence.