Two people have been rescued after their boat capsized off Granite Island near Victor Harbor.

The local sea rescue patrol retrieved the pair and towed their boat back to shore after the alarm was raised about 9pm last night.

It was the third incident on the water across South Australia on Thursday evening.

A swimmer was found safe on shore after earlier being reported missing off Kangaroo Island.

No one was injured after a ski boat overturned on the River Murray about 12 kilometres south of Mannum.