The Australian-English phrase to be able to talk under water (or under wet cement, under wet concrete) means to have the gift of the gab, i.e., to have the ability to speak with eloquence and fluency—cf. also meanings and origin of ‘blarney’.

These are the earliest occurrences of the phrase that I have found, in chronological order:

 

TO BE ABLE TO TALK UNDER WATER

 

1-: From the column The Newcastle Scene, by William Crawford, published in The Newcastle Sun (Newcastle, New South Wales) of Friday 24^th August 1951:

Overheard in passing:
“Has she the gift of the gab! Why she can talk under water!”

2-: From the column Peeps Through Keyholes, published in Truth (Brisbane, Queensland) of Sunday 20^th December 1953:

Pub group extolling virtues of various “ear-bashers” joined by stranger, who said he couldn’t help overhearing, wanted to nominate cove who worked in his Government department. “He’s good—he can talk under water,” he said.

3-: From an article about cricket, by Wally Grout, published in The Age (Melbourne, Victoria) of Saturday 1^st January 1966:

“Slash” normally a reticent sort of bloke could, when the subject is cricket, talk under water with a mouthful of sand.

4-: From Pilot-explorer’s plan for national park, by Lenore Nicklin, published in The Sydney Morning Herald (Sydney, New South Wales) of Wednesday 7^th November 1973:

Percy Trezise is an Ansett pilot based in Cairns—an historian, anthropologist, explorer, artist and writer who used to be a professional cyclist and the manager of a department store in Moonee Ponds.
He has come down from the North to launch his latest book “Last Days of a Wilderness,” to attend the opening of an exhibition of his paintings at the Artarmon Galleries (“I’m a sophisticated primitive”) and to raise money—hopefully $65,000—for his pet project, the Quinkan National Park.
“We’re already past 55,000 and we expect to have it all by next March if the public sees the light,” he said yesterday.
[…]
The Quinkan National park (“I can talk under water on this subject”) will comprise 300 sq miles of rugged sandstone country 170 miles north-west of Cairns.

 

TO BE ABLE TO TALK UNDER WET CEMENT, or WET CONCRETE

 

1-: From Sayings of the Week, published in The Sydney Morning Herald (Sydney, New South Wales) of Saturday 11^th March 1978:

Even Fred Daly said he couldn’t shut the Member for Wills up. He’d talk under wet cement.
—Mr John Bourchier, MP, on Mr Bryant.

2-: From Prime Ministers I have known, a series of anecdotal reminiscences by the political correspondent Alan Reid, published in The Bulletin (Sydney, New South Wales) of Tuesday 5^th February 1980—the following is about William McMahon (1908-1988), then Minister for Labour and National Service in the government led by Robert Gordon Menzies, Prime Minister from 19^th December 1949 to 26^th January 1966; the 1961 elections saw the Liberal-CP coalition reduced to a floor majority of one in the House of Representatives:

Cabinet was agreeing blithely to a series of proposals designed to lift the economy out of the unemployment-producing depression which the Liberal-CP coalition believed was responsible for the fright given them by the electorate. Billy McMahon intervened. McMahon was then a relatively junior minister, not that that would have bothered McMahon, of whom a contemporary once said admiringly, “Billy can talk under wet concrete.”

This photograph and caption are from The Bulletin (Sydney, New South Wales) of Tuesday 5^th February 1980:

Billy McMahon: a colleague said he could talk under wet cement

3-: From a letter published in The Bulletin (Sydney, New South Wales) of Tuesday 18^th March 1980:

A dry reply
Allan [sic] Reid quotes someone as saying “Billy McMahon could talk under wet concrete.” Two pages later, beneath a picture of the redoubtable old gentleman, an ignorant caption writer has altered the quote to “under wet cement” in the apparent belief concrete and cement are the same thing.
Not at all. Cement is an essential but quantatively [sic] minor ingredient of concrete. One imagines it would be the sheer bulk and weight of the other components, sand and metal aggregate, which would inhibit oratory in lesser politicians. Speaking under wet cement alone would be no great feat (ask any women in a mudpack). Distasteful perhaps, though not nearly as difficult as speaking through dry cement in its powdered state.
Peter Carter
Manly NSW

4-: From the column Capital Capers, by Mungo MacCallum, published in The Sun-Herald (Sydney, New South Wales) of Sunday 9^th November 1980—the following is about Michael Hodgman (1938-2013), who had just been appointed Minister for the Capital Territory in the government led by Malcolm Fraser:

Mr Hodgman is known among other things as “The Mouth from the South,” because of the fact he is a Tasmanian who is articulate to point of garrulousness, in fact, it is generally believed that he would talk under wet cement.