Was it a Great White shark attack?

A father of three was swimming just six metres from his son when he disappeared in bloody, churning waters after what is believed to have been a fatal shark attack.

The attack happened at Port Kennedy, near Rockingham, about 30 kilometres south of Perth, about 7.15am West Australian time.

Police and rescue crews, including helicopters, were this afternoon searching for the missing man, 51, who had been snorkelling for crabs when the shark closed in. His 24-year-old son did not see the attack.

The missing man has been identified as Brian Guest. It is believed he lived in an estate near the beach.

Friend Steve Kent said the family was in shock as they tried to support each other through the tragedy.

"It's a shock loss for the family. He was a loving father and husband who appears to have been taken by a shark," Mr Kent said.

"At the time he didn't sight what happened. People on the beach saw and heard things in the water."

The missing man has been described as an avid fisherman who loved the water and had great respect for its marine life.

The beach remains closed as police, surf lifesavers and council workers search the area using quad bikes, four-wheel drives and boats.

Police have asked the public to avoid the area until further notice.

Police spokesman Mark Valentine said witnesses had reported seeing "something pretty violent" happening in the water, which turned red.

"Something very traumatic has happened there and we are treating it as a probable shark attack," Mr Valentine told AAP.

"There was lots of talk among witnesses at the incident location about seeing fins in the water but we can't yet say whether there was definitely a shark out there."

The beach is a popular swimming location and there were many people out because of warm weather, Mr Valentine said.

Channel 9 reported a four-metre white pointer was spotted in the area yesterday.