Activists from the Shoreham branch of Extinction Rebellion held a silent vigil in the town on Sunday.
It came as the highest tide of the year and gale force winds and rain combined to create unusually high water levels along the River Adur.
The group said the vigil at Coronation Green symbolised ‘the drowning of people and land as sea levels rise with global warming’.
Anne Younger, a member of the Extinction Rebellion branch, said: “It was the highest spring tide of the year.
“Because of the bad weather there was a storm surge a couple of hours before high tide.
“Our photographer got a couple of photos of the water lapping onto Coronation Green, which is quite scary.
“It’s a weather event that is an example of what’s going to become more common and more extreme.”
The vigil marked the first appearance of the branch’s Red Rebel Brigade.
The Red Rebel activists dress in red, with white faces and red lips, symbolising ‘grief and love for humanity’.
A spokesman for Extinction Rebellion said: “The red reminds us of the blood being spilt, the fires of forests being torched and the anger at the loss of our future to climate collapse.”
Flood alerts have been in place in Shoreham over the last few days.
The Government’s website warned that, for an hour either side of the high tide, houseboats in the harbour would be affected and water would be high up slipways and would affect the Sussex Yacht Club, Riverside Business Centre and Surry Hard.
Today’s lunchtime tide is forecast to be ‘similar in height and impacts as Sunday lunchtime, with overnight tides 10cm lower’.
The flood gate at Oyster Quay will be closed.
Residents were advised to avoid walking through flood water, which may conceal hidden hazards.