A large south swell kept lifeguards busy on Sunday with 17 rescues and nearly 600 preventive actions along Oahu’s south shore.

The swell, forecast to produce waves 6 to 8 feet in height, is expected to build overnight Sunday and gradually diminish through Monday.

Ocean Safety officials are advising the public to know the risks before entering the water and to not test their capabilities.

In an unrelated incident, a shark sighting prompted Ocean Safety to post warning signs at Pokai Bay.

The shark, estimated at 6 to 8 feet in length, was spotted about 70 yards offshore, according to Ocean Safety.