Fears missing SA boat may have capsized

A search for a boat missing off South Australia's southeast coast is set to be scaled back amid fears it has capsized, increasing concerns for the three people on board.

The search will continue on Thursday but most likely only from the air and along the coast as fears grow for Len and Annette Vandepeer and their adult son Doug.

Police say a seat from the seven-metre boat was found on Wednesday with a bucket and a plastic tub recovered earlier.

"Unfortunately, the finding of the seat would indicate to us that the boat is no longer upright," Detective Superintendent Grant Moyle told reporters.

"The search is continuing with the hope that there is something still on the surface."

But boats will only return to the water if more items are found.

The Vandepeers' boat failed to return to Cape Jaffa on Monday night.

The search so far has centred on the waters between Beachport and The Granites, about 20km north of Kingston.

More than 2000 square nautical miles have been covered.

Three helicopters have been involved, along with State Emergency Service volunteers and local fishermen.