The British-English phrase couldn’t punch a hole in a wet Echo, and variants, are used of a weakling or of an ineffectual person.
—Synonym: couldn’t knock the skin off a rice pudding.

In this phrase, Echo refers to the Liverpool Echo, a newspaper published in Liverpool, a city and seaport historically situated in Lancashire, a county of north-western England, on the Irish Sea.

(However, cf. quotation 2, below, for a possible reference to the South Wales Echo.)

These are, in chronological order, the earliest occurrences of the phrase couldn’t punch a hole in a wet Echo, and variants, that I have found:

1-: From a letter, published in the Liverpool Echo (Liverpool, Lancashire, England) of Monday 2^nd August 1954, in which one Jenny Shearwood, of Liverpool, listed Liverpudlian phrases:

How many times has the disgusted and disappointed “kopite” ¹ been heard to remark, “Him, he couldn’t kick a hole in a wet Echo.”

¹ The noun kopite designates a supporter of Liverpool Football Club. This noun refers to the Kop, the name of a high bank of terracing for standing spectators at Anfield, home ground of Liverpool Football Club.

2-: From the memoirs of the Welsh boxer Joe Erskine (1934-1990), published in The People (London, England) of Sunday 15^th November 1964:
—Nothing in his memoirs indicates which newspaper Erskine was referring to, but it may be to the South Wales Echo, published in Cardiff, Glamorgan, Wales, since he was writing from the pub that he had just taken over, the White Hart Hotel, at Newport, near Cardiff; additionally, the boxing manager Benny Jacobs was born in, and was working in, Cardiff:

Remember when Cassius Clay ² came over last year to fight Henry Cooper ³? I was so broke I got my father to ask Jim Wicks, Cooper’s manager, if I could be Henry’s sparring partner.
That was how low I’d sunk . . . begging a few pounds from the man I had fought four times for the British heavyweight title!
Benny Jacobs nearly threw a fit when he heard. “Are you mad?” he bawled.
“What’s wrong?” I asked. “He needs help to beat Clay.”
And Benny snapped back: “Yeah, it’ll look great when everyone reads he’s knocked you out in training. And that’s what he will do. You’re not fit enough to punch a hole in a wet ‘Echo.’” (That’s our local paper.)
I shuffled my feet. Benny was right. “But I need the money, Ben.”
“How much?”
“I don’t know,” I mumbled.
“Here’s 50 quid,” he said generously. “You won’t get that for sparring.”
Perhaps I have been a fool; perhaps I have wasted money; perhaps I should have made my future secure.
But, despite everything, it’s better to have been that kind of Joe Erskine than no one.

² Muhammad Ali (Cassius Marcellus Clay – 1942-2016) was a U.S. boxer.
³ Henry Cooper (1934-2011) was a British boxer.

3-: From Frank Shaw ⁴ in Scouseland, published in the Liverpool Echo and Evening Express (Liverpool, Lancashire, England) of Saturday 5^th February 1966:

Liverpool has produced many comics and sportsmen.
Land ‘n’ Stage, delighting Norris Green audiences, are our first crosstalk act. Both are keen sportsmen.
Land, in the old Pudsey Street days ⁵, often knocked a hole in a wet Echo. And fellow-drinkers in the Blue House ⁶ will recall that Stage was a good dribbler but a bad passer.

⁴ Frank Shaw was a Liverpudlian author and journalist.
⁵ The original Liverpool Stadium was on Pudsey Street, off London Road.
⁶ The Stanley Park, known as the Blue House, is a pub near Goodison Park, Liverpool, home ground of Everton Football Club.

4-: From Reflections, a poem by one Reg Barclay, of Southgate, Runcorn, published in the Runcorn Weekly News (Runcorn, Cheshire, England) of Thursday 5^th August 1976:

They say that once upon a time,
When this chap—was in his prime.
See, his muscles, just have a decko,
Can’t punch a hole, in a wet Echo.

5-: From a letter to the Editor, published in The Guardian (London and Manchester, England) of Wednesday 19^th September 1979:

Sir,—Most treasure hunters will welcome seeing “the learned looking figures” in Britain’s fields next year—especially if they are “scattering the grass with bits of metal and ball bearings.” If that’s going to be the archaeologists’ “most forceful attempt,” treasure hunters have nothing to fear. Obviously—as they would say in Scouse Land—they “couldn’t even punch a hole in a wet Echo.”
The dropping of litter “in, on to or from any place in the open air to which the public are entitled or permitted to have access without payment” is an offence under the Litter Act. On privately owned land the position is, of course, different—but can anyone imagine a farmer consenting to bits of iron being scattered over his pasture? The shotguns would be out with a vengeance, although the idea of having stuffed professors of archaeology on the wall instead of the more usual trophies would have some appeal.—Yours faithfully,
J. C. Williams.
60A High Street,
Edgware, Middx.

6-: From an article by Michael Charters, published in the Liverpool Echo (Liverpool, Merseyside, England) of Tuesday 26^th August 1980—Robert Paisley (1919-1996) was then the manager of Liverpool Football Club:

Mr. Paisley, disgusted with their performance at Leicester on Saturday, has demanded a complete change of attitude from his stars. “If they don’t show it at Bradford, there’ll be changes. I won’t permit this to go on any longer than that,” he said.
Mr. Paisley has drummed home that message to the players yesterday and to-day. They’re in no doubt where they stand—it’s improve or else.
He told me to-day: “They couldn’t have punched a hole in a wet Echo last Saturday. Their attitude was all wrong, strolling around as though it was easy.”