A man and his five-year-old son, who both drowned in an Airlie Beach public pool, were underwater for about six minutes before anyone noticed, Queensland Police say.

Bystanders and police have described the "frantic" efforts to save the boy and his 30-year-old father, who were pulled unconscious from the water at the Airlie Beach lagoon about 4:00pm on Sunday.

Inspector Steve O'Connell said CCTV of the pool showed the man, a Chinese tourist who couldn't swim, entering the pool with his son at the shallow end.

"The father's moved along into slightly deeper water with the son on his back and he's gone into slightly more deeper water and from there both of them have gone under," Inspector O'Connell said.

"He's tried to raise the alarm but it wasn't sufficient for anyone that was around him to notice the distress that they were in or that they were sinking to the bottom.

"Unfortunately despite it being a crowded situation on a hot, busy day in the lagoon, no-one has noticed the circumstances unfolding until some, approximately, six minutes after they went under."

He said when someone swimming noticed the man at the bottom of the pool the alarm was raised and lifeguards pulled him from the water.

David Muir was at the pool when he noticed the father being dragged from the water by lifeguards.

He said he helped perform CPR on the man.

Witness and first responder David Muir said he was devastated by what happened. ( ABC News: Tara Cassidy )

"We'd maybe been going for five minutes working on the father and I heard a voice to my right a lady shout out, 'is that boy still under the water?'" he said.

"When I turned around I saw a child fully submerged in the water and I was in two minds do I continue or do I leave and abort."

Inspector O'Connell said the boy was pulled from the water a few minutes after the man and there was a "frantic" effort to resuscitate them.

He said the man's wife was at the pool at the time.

"After the father and son have gone into the water, mum has left the lagoon area briefly to get a drink. When she's returned the tragedy is unfolding before her eyes," Inspector O'Connell said.

He said the woman was "exceptionally distraught" and was being helped by the Chinese embassy.

He said it was a tragic set of circumstances.

The lagoon remains closed until further notice. ( ABC News )

"That you can drown in a crowded public area like that, but the CCTV clearly shows it was not something that anyone around them could pick up on," he said.

"With it being so crowded and very active and noisy at the time it was several minutes before they were noticed on the bottom."

Jodie Ginger was at the lagoon with her two children on Sunday afternoon when people began performing CPR on the pair.

She said the scene would stay with her forever.

With her two girls shaken from the event, she said she just went home and cried.

"Our little community is crying for them," Ms Ginger said.

David Muir said he was devastated by what happened.

"We worked for 40 minutes to bring that gentleman back … but sadly it was too little too late."

The lagoon was closed on Monday.

The Airlie Beach lagoon is patrolled by lifeguards and is a popular destination for locals and visitors.

Whitsunday Mayor Andrew Willcox praised the efforts of everyone who came to their aid and tried to resuscitate them.

"Obviously it was extremely distressing for everyone involved, both police, emergency services and civilians," he said.

He said the council would offer counselling to those involved.