A search and rescue operation for a teenage boy swept off the coast of South Australia's popular tourist town, Victor Harbor, will continue on Monday.

Emergency crews including South Australian Police, PolAir and the local sea rescue squadron attended an area close to Granite Island on Sunday.

SA Police say the alarm was raised about 3.15pm after the boy was seen to fall into the water.

The Metropolitan Fire Service (MFS) was originally called to assist.

An MFS spokesperson said the boy had been swept out and other swimmers tried to assist, but had to abandon their efforts.

Witness Akash, who was at the scene, said he saw a mother crying and shouting.

"No one had any idea what was happening — but something is not right," Akash said.

"The police and the rescue team are here — I think the [parents] are scared… the child is missing.

"We are praying."

Officer in charge of the Hills Fleurieu Local Service Area, Superintendent Joanne Howard, said the search will continue on Monday.

"We will continue to search with the assistance of local emergency services and other resources," she said.

"We would like to thank the local community and emergency services for their assistance."